pgsql: doc: add docs for age(xid) and mxid_age(xid)

doc: add docs for age(xid) and mxid_age(xid)

Reported-by: David Rowley

Discussion: /messages/by-id/CAKJS1f_OQpz7rpe-KJmskVxbU06buiXbfonxG3JLB+nGCJ5E=g@mail.gmail.com

Backpatch-through: 16

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/48b5aa31437a161356fb8ec820b81b7b67cf04c2

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