psql: Include \pset xheader_width in --help=commands|variables
psql's --help was missed the description of the \pset variable
xheader_width, that should be listed when using \? or --help=commands,
and described for --help=variables.
Oversight in
a45388d6e098.
Author: Pavel Luzanov
Discussion: https://postgr.es/m/
1e3e06d6-0807-4e62-a9f6-
c11481e6eb10@postgrespro.ru
Backpatch-through: 16