pgsql: doc: Add description of some missing monitoring functions

doc: Add description of some missing monitoring functions

This commit adds some documentation about two monitoring functions:
- pg_stat_get_xact_blocks_fetched()
- pg_stat_get_xact_blocks_hit()

The description of these functions has been removed in ddfc2d9, later
simplified by 5f2b089, assuming that all the functions whose
descriptions were removed are used in system views. Unfortunately, some
of them were are not used in any system views, so they lacked
documentation.

This gap exists in the docs for a long time, so backpatch all the way
down.

Reported-by: Michael Paquier
Author: Bertrand Drouvot
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/ZBeeH5UoNkTPrwHO@paquier.xyz
Backpatch-through: 11

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3ba8bce938154013a2561aca3f31f13d1ec11996

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 28 ++++++++++++++++++++++++++++
1 file changed, 28 insertions(+)