pgsql: doc: Fix section of functions age(xid) and mxid_age(xid)

doc: Fix section of functions age(xid) and mxid_age(xid)

In 17~, age(xid) and mxid_age(xid) were listed as deprecated. Based on
the discussion that led to 48b5aa3143, this is not intentional as this
could break many existing monitoring queries. Note that vacuumdb also
uses both of them.

In 16, both functions were listed under "Control Data Functions", which
is incorrect, so let's move them to the list of functions related to
transaction IDs and snapshots.

Author: Bertrand Drouvot
Discussion: https://postgr.es/m/Zzr2zZFyeFKXWe8a@ip-10-97-1-34.eu-west-3.compute.internal
Discussion: https://postgr.es/m/20231114013224.4z6oxa6p6va33rxr@awork3.anarazel.de
Backpatch-through: 16

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1c99d8186fbf3a55de4d7b6dd03a1a0ba4ed8997

Modified Files
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doc/src/sgml/func.sgml | 58 ++++++++++++++++++++++++++------------------------
1 file changed, 30 insertions(+), 28 deletions(-)