doc: clarify default checksum behavior in non-master branches

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