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

The following discussion from !23217 (merged) should be addressed:

• @dosuken123 started a discussion: (+2 comments)

  This is to be expanded in the future. Merge request pipeline's refs should be MergeRequest#ref_path. "refs/#{Repository::REF_MERGE_REQUEST}/#{iid}/head". This requires gitlab-runner!1092 (closed) runner fix.