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