flake8: Add comments in config to explain suppressed checks

Change-Id: Ib5c09b36d40a96ba9167b42b3bd2f1ed072660b7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254611


Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>