doc/rados: improve pg_num/pgp_num info
Improve the guidance around setting pg_num, and clear up confusion
around whether pgp_num should be set manually or, indeed, if it even can
be set manually.
This PR was raised in response to Mark Schouten's email here: https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/thread/CBDJTLTTIEZVG7GVZBX37UAWGYNSSMPD/
Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit
c43e7337212fe38e8db63d00345fa9858b3cb10a)