doc/rados: update PG guidance

doc/rados: update PG guidance

Update the "Creating a Pool" section of doc/rados/operations/pools.rst
so that the documentation no longer insists that the user change the
values of "osd_pool_default_pg_num" and "osd_pool_default_pgp_num".

See also: https://github.com/ceph/ceph/pull/55419

Tracker: https://tracker.ceph.com/issues/64259

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit 5ad241442d2c141ba508faba61f39d70f3f09679)