Refer readers of \? to "\? variables" for pset options

Refer readers of \? to "\? variables" for pset options

... and remove the list of \pset options from the general \? output.
That list was getting out of hand, both for developers to keep up to
date as well as for users to read.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/202511041638.dm4qukcxfjto@alvherre.pgsql