doc/rados/configuration: Mention show-with-defaults and ceph-conf

doc/rados/configuration: Mention show-with-defaults and ceph-conf

A small improvement based on
"Why is it still so difficult to just dump all config and where it comes from?"
https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/thread/EZSLRYBYEWDA6YIARQVMUKQUWHAE3PGR/

`show-with-defaults` is very useful, and `ceph-conf` is mentioned
so that it's clear that it's legacy, and the user doesn't have to
wonder if it's actually useful but was forgotten in the list.

Signed-off-by: Niklas Hambüchen <mail@nh2.me>