We check every so often that the document you are looking at hasn't changed. When someone supersedes the version you are looking at, or maybe even deletes it, we let you know. Also, if someone is simultaneously making comments you will probably find it useful to know.

You can refresh your window to show the changed version, though you may get told again if the same person continues to make comments. Or you can just take note and not be bugged again about that page.