doc/cephfs: edit add-remove-mds

doc/cephfs: edit add-remove-mds

Disambiguate a note in doc/cephfs/add-remove-mds.rst to help readers
distinguish between cases in which they might want to use an automated
tool such as cephadm to deploy MDSes and cases in which they might want
to manually deploy MDSes.

See: https://github.com/ceph/ceph/pull/45639

Tracker: https://tracker.ceph.com/issues/54551

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