I have changed the status of commitfest entry to "Returned with Feedback" as Laurenz's comments have not yet been resolved. Please handle the comments and update the commitfest entry accordingly.
Here's another attempt, applying Laurenz's feedback:
I removed all changes to the SELECT documentation. That might seem strange given that the heavy lifting happens in the SELECT, but I'm working from the assumption that people's greatest need for a ctid self-join will be because they are trying to find the LIMIT keyword on UPDATE/DELETE and coming up empty.
Because the join syntax is subtly different between UPDATE and DELETE, I've kept code examples in both, but the detailed explanation is in UPDATE under the anchor "update-limit" and the DELETE example links to it.