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

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  (commit 48b5aa3143)
Author: Bruce Momjian  (commit 48b5aa3143)
Co-authored-by: Bertrand Drouvot  (commit 15afb7d61c)
Reviewed-by: Michael Paquier  (commit 15afb7d61c)
Reviewed-by: Robert Haas 
Reviewed-by: Álvaro Herrera 
Discussion: https://postgr.es/m/aGMCxHxLfeMdQk8m%40nathan
Backpatch-through: 13-15

Branch
------
REL_13_STABLE

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

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