consistently document the configuration option of each module

[ forked off T917 ]

While we are at it, it is nowadays probably pointless to document configuration in README. Rather, we should consistently store configuration information in the python documentation of each module, so that we can cross-reference from one module to another. For instance, instead of (re-)documenting the storage configuration requirements in the Git loader module, we should just point to the corresponding configuration documentation of the storage module.

How about a top-level "configuration" document in each module?

---

Migrated from T1758 (view on Phabricator)