Add comment mentioning that the author's correct group could not be determined

Following from !390 (comment 278719721), some MRs may not have group label applied because the author's correct group could not be determined.

This may happen in the following scenarios:

• MR author is a gitlab team member:
  • The MR author's group is not listed in www-gitlab-com data/team.yml under departments
  • The MR author may belong to more than one teams (e.g member of one team and intern in another)
• MR author is a community member

/cc @gl-quality/eng-prod