I set this idea aside during the final v13 commitfest, but I figure that it's fine to work on documentation improvements during feature freeze, so I'm going to try to push it forward over the next few weeks.
If it's ok to work on doc patches during the feature freeze, and if we're already tweaking function documentation, would it be possible to add in anchor ids to function definitions so that we could reference specific functions (or rather the family of functions that share a name like this: https://www.postgresql.org/docs/devel/functions-datetime.html#FUNCTION-DATE-PART or similar. I tried it out just now, and the anchoring works, but there's no obvious place to acquire the anchored link, so presumably we'd anchor-ize the function name itself.
