pgsql: Rename the WAIT FOR command to WAIT in the documentation

Rename the WAIT FOR command to WAIT in the documentation

The command tag is WAIT, but the reference page called the command
"WAIT FOR" in its title, index entry, psql help name and throughout its
prose, and so did the error messages. FOR is a noise word that makes the
grammar read like English, exactly as INTO does for INSERT and FROM does
for DELETE; neither of those is part of the command's name. Settle on
WAIT everywhere the command is named, and keep WAIT FOR LSN wherever the
syntax itself is being shown.

Rename the reference page's id and file name to match, following
cb2c696b1dd: v19 is not released, so the URL is not yet baked into a
supported version, and every other page's id matches its title.

Reported-by: Masahiko Sawada sawada(dot)mshk(at)gmail(dot)com
Author: Xuneng Zhou xunengzhou(at)gmail(dot)com
Reviewed-by: Alexander Korotkov aekorotkov(at)gmail(dot)com
Discussion: https://postgr.es/m/CAD21AoBdtiPTbm7T_aNeDON9JpFG9g%3DJDUP4WU-Y8t8_xqvR5Q%40mail.gmail.com
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/a23ab4862cfebb9aba58f713854b7d5e42d5f947

Modified Files
--------------
doc/src/sgml/high-availability.sgml | 2 +-
doc/src/sgml/ref/allfiles.sgml | 2 +-
doc/src/sgml/ref/{wait_for.sgml => wait.sgml} | 30 +++++++++++++--------------
doc/src/sgml/reference.sgml | 2 +-
src/backend/commands/wait.c | 8 +++----
src/test/recovery/t/049_wait_for_lsn.pl | 9 ++++----
6 files changed, 26 insertions(+), 27 deletions(-)