pgsql: Document age(xid) and mxid_age(xid).

Document age(xid) and mxid_age(xid).

These functions have been around for some time, but commits
48b5aa3143 and 15afb7d61c were only back-patched to v16. Let's
back-patch them to all supported versions now.

Reported-by: David Rowley <dgrowleyml(at)gmail(dot)com> (commit 48b5aa3143)
Author: Bruce Momjian <bruce(at)momjian(dot)us> (commit 48b5aa3143)
Co-authored-by: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com> (commit 15afb7d61c)
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz> (commit 15afb7d61c)
Reviewed-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Reviewed-by: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Discussion: https://postgr.es/m/aGMCxHxLfeMdQk8m%40nathan
Backpatch-through: 13-15

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4bcca5ff829ba20c2d4a707c02a8e602b9ad5871

Modified Files
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doc/src/sgml/func.sgml | 31 ++++++++++++++++++++++++++++++-
1 file changed, 30 insertions(+), 1 deletion(-)