command: add a `repo help` tip to --help output

For people used to running `repo xxx --help`, they might not realize
that there are detailed man pages behind `repo help xxx`.  Add a note
to all --help commands to improve discoverability.

Change-Id: I84af58aa0514cc7ead185f6c2534a8f88e09a236
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255853
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>