Add a comment on `Ci::Pipeline#git_ref` about merge request ref

What does this MR do?

This is one of the follow-ups from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23217

What are the relevant issue numbers?

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/54908

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer