Running './gerrit_client.py' currently shows only "help" as an available
subcommand. Adding these docstrings allows it to properly show all the
other subcommands.

R=sokcevic@google.com

Change-Id: I44c9d63fe8d481cabade50311d0f630b0f747018
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3032207
Auto-Submit: Michael Moss <mmoss@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>