No discussion of custom variables; no "See also" for set_config, current_setting, pg_settings

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/14/sql-set.html
Description:

A reader of this section
https://www.postgresql.org/docs/current/sql-set.html of the documentation
might be forgiven for thinking that Postgres does not support custom
variables.

There are also related commands and features that are not mentioned. These
might be covered by a reference to 20.1.3, but that section also does not
mention custom variables afaict.

I'd be happy to write some better documentation (for all the sections in
question). I think actually a separate section under 20.1 that could be
referred to from SET, SHOW etc might be best.

I've contributed to the documentation before, but I forget how. A
description of how at the bottom of the Documentation home page or at least
in the FAQ might be in order also.