The Report extension adds a mechanism for users to privately report revisions of pages for admin attention, as well as mechanism for admins to handle such reports.
Finding a revision
First, find the problematic revision that needs attention. There will be a link to report it on either the diff:
Or on the history entry:
Once found, click the link.
Filling out report
Clicking the report link will bring the user to Special:Report.
Fill out the reason that this revision is being reported, then click "Submit report".
An admin will eventually handle the report. It is their decision whether to notify the reporter, as well as what timeframe to handle reports in.
Clicking that link will bring the user to Special:HandleReports.
Clicking a "view report" link will bring the user to view that report:
After taking any actions that might be required, such as hiding revisions, posting talk page messages, etc., making use of the links and information displayed, click "Mark as handled". The user will be redirected back to Special:HandleReports to take care of any other outstanding reports.