pgsql: pgstat: Update docs to match the shared memory stats reality.

pgstat: Update docs to match the shared memory stats reality.

This includes removing documentation for stats_temp_directory, adding
documentation for stats_fetch_consistency, rephrasing references to the stats
collector and documenting that starting a cleanly shut down standby will not
remove stats anymore. The latter point might require further wordsmithing, it
wasn't easy to adjust some of the existing content.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Reviewed-By: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-By: "David G. Johnston" <david.g.johnston@gmail.com>
Reviewed-By: Lukas Fittl <lukas@fittl.com>
Discussion: https://postgr.es/m/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b3abca68106d518ce5d3c0d9a1e0ec02a647ceda

Modified Files
--------------
doc/src/sgml/backup.sgml            |   2 -
doc/src/sgml/catalogs.sgml          |   6 +-
doc/src/sgml/config.sgml            |  49 ++++++++-----
doc/src/sgml/func.sgml              |   4 +-
doc/src/sgml/glossary.sgml          |  38 +++++-----
doc/src/sgml/high-availability.sgml |  13 ++--
doc/src/sgml/maintenance.sgml       |   2 +-
doc/src/sgml/monitoring.sgml        | 142 +++++++++++++++++++-----------------
doc/src/sgml/ref/pg_dump.sgml       |   2 +-
9 files changed, 139 insertions(+), 119 deletions(-)