Rake for_more_information checks should use full URLs

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In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3553#note_50200671, I see that when there is a warning message, we call for_more_information('doc/gitlab-geo/README.md'), which prints out:

 For more information see:
  doc/gitlab-geo/README.md

For most users, this relative path isn't really useful. We should have a link to GitLab.com that takes you exactly what you need to know.

@axil Do you think this is a good idea?