doc/rados: update config for autoscaler

doc/rados: update config for autoscaler

Update doc/rados/configuration/pool-pg-config-ref.rst to account for the
behavior of autoscaler.

Previously, this file was last meaningfully altered in 2013, prior to
the invention of autoscaler. A recent confusion was brought to my
attention on the Ceph Slack whereby a user attempted to alter the
default values of a Quincy cluster, as suggested in this documentation.
That alteration caused Ceph to throw the error "Error ERANGE: 'pgp_num'
must be greater than 0 and lower or equal than 'pg_num', which in this
case is one" and a related "rgw_init_ioctx ERROR" reading in part
"Numerical result out of range". The user removed the
"osd_pool_default_pgp_num" configuration line from ceph.conf and the
cluster worked as expected. I presume that this is because the removal
of this configuration line allowed autoscaler to work as intended.

Fixes: https://tracker.ceph.com/issues/64259
Co-authored-by: David Orman <ormandj@corenode.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit 4dc12092be584da44baca14e31ca33231164235f)