pgsql: psql: Include \pset xheader_width in --help=commands|variables

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: /messages/by-id/1e3e06d6-0807-4e62-a9f6-c11481e6eb10@postgrespro.ru
Backpatch-through: 16

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4ecb136bfaa72780332c495259d99ffd236da50e

Modified Files
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src/bin/psql/help.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)