doc/cephfs/add-remove-mds: added cephadm note, refined "Adding an MDS"
Description: 1) Add a note about using cephadm for setting up the
cluster and mds(s), also mention the use of ceph
orchestrator if one needs to setup mds(s) manually.
2) Changed the term `data point` to `directory` in
point 1 under "Adding an MDS" section for better
clarity.