BUG #15909: Doc for `initdb` on `--data-checksums` says "cannot be changed later", but *can* be in Pg 12.

The following bug has been logged on the website:

Bug reference:      15909
Logged by:          Basil Bourque
Email address:      basil.bourque@gmail.com
PostgreSQL version: 12beta2
Operating system:   macOS
Description:

The `--data-checksums` option on this page for `initdb`:
https://www.postgresql.org/docs/12/app-initdb.html

…needs to be updated. 

Currently says "This option can only be set during initialization, and
cannot be changed later."

But now in Postgres 12, the `pg_checksums` command can enable or disable
checksums on an existing cluster.

See: https://www.postgresql.org/docs/12/app-pgchecksums.html

--Basil Bourque