developer-setup: update system dependencies

Add rsync which was implicit dependency for docker dev setup.
Move comments so that the command is runnable as-is.

docs/developer-setup.rst

@@ -30,17 +30,18 @@ package manager [1]. On Debian/Ubuntu-based distributions::
   echo "deb https://downloads.apache.org/cassandra/debian 40x main" | sudo tee -a /etc/apt/sources.list.d/cassandra.list
   sudo apt update
   sudo apt install \
-      python3 python3-pip python3-venv virtualenvwrapper \ # Python dev tools
-      build-essential pkg-config lzip \                    # basic system utilities
-      libpython3-dev libsystemd-dev libsvn-dev \           # Python libraries build deps
-      git myrepos \                                        # manage repositories
-      postgresql libpq-dev \                               # main database
-      cassandra \                                          # alternative main database
-      postgresql-autodoc graphviz plantuml inkscape \      # for documentation
-      fuse3 libfuse3-dev                                   # FUSE interface
+      build-essential pkg-config lzip rsync \
+      python3 python3-pip python3-venv virtualenvwrapper \
+      libpython3-dev libsystemd-dev libsvn-dev fuse3 libfuse3-dev \
+      git myrepos \
+      postgresql-autodoc graphviz plantuml inkscape \
+      postgresql libpq-dev cassandra
 
 [1] python3.7 runtime version is the minimum required
 
+This installs basic system utilities, Python library dependencies, development tools,
+documentation tools and our main database management systems.
+
 Cassandra and postgresql will be started by tests when they need it, so you
 don't need them started globally (this will save you some RAM)::