doc/rados: document manually passing search domain

doc/rados: document manually passing search domain

Document how to manually pass the search domain to "mon_dns_srv_name" in
doc/rados/configuration/mon-lookup-dns.rst.

This commit is made in response to a request by Lander Duncan that was made on the [ceph-users] mailing list, and can be seen here: https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/thread/F7V4CWLIYCAJ4JXI2JLNY6QPCFPR4SLA/

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>