doc: clarify default checksum behavior in non-master branches
Also simplify and correct data checksum wording in master now that it is
the default. PG 13 did not have the awkward wording.
Reported-by: Felix <afripowered@gmail.com>
Reviewed-by: Laurenz Albe
Discussion: https://postgr.es/m/
173928241056.707.
3989867022954178032@wrigleys.postgresql.org
Backpatch-through: 14