repo/main: add module docstrings

This should help people get some bearings in the codebase.

Change-Id: I951238fe617a3ecb04a47ead3809ec72c8fbf5a1
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/231232
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>