Be more helpful when pointing out issues #869

In git/git#1207 (comment), I felt the need to explain why a noreply.github.com email address is not good enough. Which makes me believe that GitGitGadget could be taught to be more helpful in the first place.

We probably want to vet all linter messages, imagining ourselves as new contributors to the project who do not know anything about the customs of the Git project. If any linter message would leave such a new contributor behind even more puzzled than before, it is safe to assume that the message can be easily improved.