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