It is there actually, but only in the table on the beginning:
But I agree, it is easy to miss, I myself was in a similar situation not so long ago, having missed the flat suffix.
Thanks to @pmario we have a much better organized filter run prefix docs since recently, with the nice railroad diagrams. But map seems to be missing the suffix in its diagram, I’ll fix it. I think it will be much better visible then.
Edit: I made docs PR Improve filter run prefix docs by mateuszwilczek · Pull Request #8067 · TiddlyWiki/TiddlyWiki5 · GitHub