fix deprecation mention for age() and mxid_age()

Hi hackers,

48b5aa3143 mentioned age() and mxid_age() as deprecated. As per the discussion in
[1], it looks like that was not intentional.

Please find attached a patch to fix this on master and its version for the 17
stable branch backpatch. I think that's worth a backpatch as that could raise
concerns to read that the functions you rely on for monitoring are deprecated
and could generate questions like "will they be removed in PG 18?", "if so, what
is the supported/recommended way to calculate age of transaction id’s?".

Also it looks like that for PG 16, 7471230cc6, did put the functions description
in the "Control Data Functions". That looks unintentional too, so moving it to
"Transaction ID and Snapshot Information Functions" section instead.

Please find attached the 3 patches.

[1]: https://www.postgresql.org/message-id/20231114013224.4z6oxa6p6va33rxr%40awork3.anarazel.de

Looking forward to your feedback,

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com