README: Update instructions to configure and run datastore scrubbing

Promote use of `swh scrubber check run` instead of deprecated commands.

Add sections about object storage checker and journal checker.

README.rst

 Software Heritage - Datastore Scrubber
 ======================================
 
-Tools to periodically checks data integrity in swh-storage and swh-objstorage,
-reports errors, and (try to) fix them.
+Tools to periodically checks data integrity in ``swh-storage``, ``swh-objstorage``
+and ``swh-journal``, reports errors, and (try to) fix them.
 
-This is a work in progress; some of the components described below do not
-exist yet (cassandra storage checker, objstorage checker, recovery, and reinjection)
 
 The Scrubber package is made of the following parts:
 
@@ -18,7 +16,7 @@ compute checksums, and write any failure in a database, along with the data of
 the corrupt object.
 
 There is one "checker" for each datastore package: storage (postgresql and cassandra),
-journal (kafka), and objstorage.
+journal (kafka), and object storage (any backends).
 
 The journal is "crawled" using its native streaming; others are crawled by range,
 reusing swh-storage's backfiller utilities, and checkpointed from time to time
@@ -32,26 +30,75 @@ able to spawn a number of checkers.
 
 A new configuration is created using the ``swh scrubber check init`` tool:
 
-.. code-block:: bash
+.. code-block:: console
 
-   $ swh scrubber check init --object-type snapshot --nb-partitions 65536 --name chk-snp
+   $ swh scrubber check init storage --object-type snapshot --nb-partitions 65536 --name chk-snp
    Created configuration chk-snp [2] for checking snapshot in datastore storage postgresql
 
+.. note::
+
+   A configuration file is expected, as for most ``swh`` tools.
+   This file must have a ``scrubber`` section with the configuration of
+   the scrubber database. For storage checking operations, this
+   configuration file must also have a ``storage`` configuration section.
+   See the `swh-storage documentation`_ for more details on this. A
+   typical configuration file could look like:
+
+   .. code-block:: yaml
+
+      scrubber:
+        cls: postgresql
+        db: postgresql://localhost/postgres?host=/tmp/tmpk9b4wkb5&port=9824
+
+      storage:
+        cls: postgresql
+        db: service=swh
+        objstorage:
+          cls: noop
+
+.. note::
+
+   The configuration section ``scrubber_db`` has been renamed as
+   ``scrubber`` in ``swh-scrubber`` version 2.0.0
+
 One (or more) checking worker can then be spawned by using the ``swh scrubber
-check storage`` command:
+check run`` command:
 
-.. code-block:: bash
+.. code-block:: console
 
-   $ swh scrubber check storage chk-snp
+   $ swh scrubber check run chk-snp
    [...]
 
 
-.. note:: A configuration file is expected, as for most ``swh`` tools.
-          This file must have a ``scrubber`` section with the configuration of
-          the scrubber database. For storage checking operations, this
-          configuration file must also have a ``storage`` configuration section.
-          See the `swh-storage documentation`_ for more details on this. A
-          typical configuration file could look like:
+Object storage
+++++++++++++++
+
+As with the storage checker, a checking configuration must be created before
+being able to spawn a number of checkers.
+
+A new configuration is created using the ``swh scrubber check init`` tool:
+
+.. code-block:: console
+
+   $ swh scrubber check init objstorage --object-type content --nb-partitions 65536 --name check-contents
+   Created configuration check-contents [3] for checking content in datastore objstorage remote
+
+.. note::
+
+   A configuration file is expected, as for most ``swh`` tools.
+   This file must have a ``scrubber`` section with the configuration of
+   the scrubber database. For object storage checking operations, this
+   configuration file must have:
+
+   - a ``storage`` configuration section if content ids are read from it (default)
+   - a ``journal`` configuration section if content ids are read from a kafka content
+     topic (require to use flag ``--use-journal`` of the ``swh scrubber check run``
+     command)
+   - an ``objstorage`` configuration section targeting the object storage to check
+
+   See the `swh-storage documentation`_, `swh-objstorage documentation`_ and
+   `swh-journal documentation`_ for more details on this. A typical configuration
+   file could look like:
 
    .. code-block:: yaml
 
@@ -65,10 +112,91 @@ check storage`` command:
         objstorage:
           cls: noop
 
-.. _`swh-storage documentation`: https://docs.softwareheritage.org/devel/swh-storage/index.html
+      journal:
+         cls: kafka
+         brokers:
+            - broker1.journal.softwareheritage.org:9093
+            - broker2.journal.softwareheritage.org:9093
+            - broker3.journal.softwareheritage.org:9093
+            - broker4.journal.softwareheritage.org:9093
+         group_id: swh.scrubber
+         prefix: swh.journal.objects
+         on_eof: stop
+
+      objstorage:
+        cls: remote
+        url: https://objstorage.softwareheritage.org/
+
+By default, an object storage checker detects missing and corrupted contents.
+To disable detection of missing contents, use the ``--no-check-references``
+option of the ``swh check init`` command.
+To disable detection of corrupted contents, use the ``--no-check-hashes``
+option of the ``swh check init`` command.
+
+One (or more) checking worker can then be spawned by using the ``swh scrubber
+check run`` command:
+
+- if the content ids must be read from a storage instance
+
+.. code-block:: console
 
-.. note:: The configuration section ``scrubber_db`` has been renamed as
-          ``scrubber`` in ``swh-scrubber`` version 2.0.0
+   $ swh scrubber check run check-contents
+   [...]
+
+- if the content ids must be read from a kafka content topic of ``swh-journal``
+
+.. code-block:: console
+
+   $ swh scrubber check run check-contents --use-journal
+   [...]
+
+Journal
++++++++
+
+As with the other checkers, a checking configuration must be created before being
+able to spawn a number of checkers.
+
+A new configuration is created using the ``swh scrubber check init`` tool:
+
+.. code-block:: console
+
+   $ swh scrubber check init journal --object-type directory --name check-dirs-journal
+   Created configuration check-dirs-journal [4] for checking directory in datastore journal kafka
+
+.. note::
+
+   A configuration file is expected, as for most ``swh`` tools.
+   This file must have a ``scrubber`` section with the configuration of
+   the scrubber database. For journal checking operations, this
+   configuration file must also have a ``journal`` configuration section.
+
+   See the `swh-journal documentation`_ for more details on this.
+   A typical configuration file could look like:
+
+   .. code-block:: yaml
+
+      scrubber:
+        cls: postgresql
+        db: postgresql://localhost/postgres?host=/tmp/tmpk9b4wkb5&port=9824
+
+      journal:
+         cls: kafka
+         brokers:
+            - broker1.journal.softwareheritage.org:9093
+            - broker2.journal.softwareheritage.org:9093
+            - broker3.journal.softwareheritage.org:9093
+            - broker4.journal.softwareheritage.org:9093
+         group_id: swh.scrubber
+         prefix: swh.journal.objects
+         on_eof: stop
+
+One (or more) checking worker can then be spawned by using the ``swh scrubber
+check run`` command:
+
+.. code-block:: console
+
+   $ swh scrubber check run check-dirs-journal
+   [...]
 
 Recovery
 --------
@@ -86,3 +214,7 @@ Reinjection
 
 Finally, when an original object is recovered, it is reinjected in the original
 data store, replacing the corrupt one.
+
+.. _`swh-storage documentation`: https://docs.softwareheritage.org/devel/swh-storage/index.html
+.. _`swh-objstorage documentation`: https://docs.softwareheritage.org/devel/swh-objstorage/index.html
+.. _`swh-journal documentation`: https://docs.softwareheritage.org/devel/swh-journal/index.html