Simplify description of --data-checksums in documentation of initdb

Simplify description of --data-checksums in documentation of initdb

The documentation mentioned that data checksums cannot be changed after
initialization, which is not true as pg_checksums can do that with its
--enable option introduced in v12.  This simply removes the sentence
telling so.

Reported-by: Basil Bourque
Author: Michael Paquier
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/15909-e9d74271f1647472@postgresql.org
Backpatch-through: 12