pgsql: doc: Add more details about pg_stat_get_xact_blocks_{fetched,hit

doc: Add more details about pg_stat_get_xact_blocks_{fetched,hit}

The explanation describing the dependency to system read() calls for
these two functions has been removed in ddfc2d9. And after more
discussion about d69c404, we have concluded that adding more details
makes them easier to understand.

While on it, use the term "block read requests" (maybe found in cache)
rather than "buffers fetched" and "buffer hits".

Per discussion with Melanie Plageman, Kyotaro Horiguchi, Bertrand
Drouvot and myself.

Discussion: https://postgr.es/m/CAAKRu_ZmdiScT4q83OAbfmR5AH-L5zWya3SXjaxiJvhCob-e2A@mail.gmail.com
Backpatch-through: 11

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1372166bac300f9d394fbfde7b7ff0ecebfa7e48

Modified Files
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doc/src/sgml/monitoring.sgml | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)