Feature summary:
When viewing a diff from the revision history, such as https://www.mediawiki.org/w/index.php?diff=3878227, there should be a help icon at the top-right linking to mw:Help:Diff, just like there is at Special:Diff.
Use case(s):
Viewing diffs via the revision history (or other change lists such as Special:RecentChanges) is likely the most common way to view diffs. By comparison Special:Diff is rarely used. Thus most users will never find the documentation.
Benefits:
Users should have access to documentation through the pages they're most likely to use.