Capture recent merge refs changes in docs

From this discussion (https://gitlab.slack.com/archives/C01EMBKS5DW/p1612996524311800) in team chat, we have a small mention of merge refs in https://docs.gitlab.com/ee/development/diffs.html#merge-request-diffs-against-the-head-of-the-target-branch that may need to be expanded either in the current (dev-focused) page, or in a new page more focused on the end user.

&854 (closed) is the epic.
@garyh notes the docs need to be cleaned up because the base version will be gone, and that "[a] casual user shouldn't really notice anything has changed, hopefully. Maybe mention that they are now more accurate as of 13.9?"
https://docs.gitlab.com/ee/user/project/merge_requests/reviewing_and_managing_merge_requests.html was described as a possible home, but this page is overloaded already.
@phikai notes https://docs.gitlab.com/ee/user/project/merge_requests/reviewing_and_managing_merge_requests.html#view-changes-between-file-versions might be a good home, starting in that section. He observed that most people didn't see diffs change for gitlab-org/gitlab or gitlab-com/www-gitlab-com over the last few weeks, but those who did perceive it as a significant problem.
!48445 (closed) was created to address the topic, but was closed without merging. It may contain useful starting info.