Note that do_help is used when short messages might be wanted

Note that do_help is used when short messages might be wanted

From http://marc.info/?l=git&m=122718711327376&w=2, by Petr:

  Note that I think these changes are reasonable only as long as do_help
  is just a short two-line usage help; in the event we also implement a
  longer help, it certainly shouldn't be printed in these cases.

This commit adds a comment to the do_help function to remind us.

Signed-off-by: martin f. krafft <madduck@debian.org>