Update docs to say that external Ceph required

There is no longer support for provisioning Ceph in Kolla Ansible, so we
should no longer say that it's only sometimes necessary to create the
cluster/pools/keyrings externally.

Change-Id: Ia3026cfeebfb8258b79490f9facc341c928845f9

doc/source/reference/storage/external-ceph-guide.rst

@@ -4,9 +4,16 @@
 External Ceph
 =============
 
-Sometimes it is necessary to connect OpenStack services to an existing Ceph
-cluster instead of deploying it with Kolla. This can be achieved with only a
-few configuration steps in Kolla.
+Kolla Ansible does not provide support for provisioning and configuring a
+Ceph cluster directly. Instead, administrators should use a tool dedicated
+to this purpose, such as:
+
+* `ceph-ansible <https://docs.ceph.com/ceph-ansible>`_
+* `ceph-deploy <https://docs.ceph.com/docs/master/start/>`_
+* `cephadm <https://docs.ceph.com/docs/master/bootstrap/>`_
+
+The desired pool(s) and keyrings should then be created via the Ceph CLI
+or similar.
 
 Requirements
 ~~~~~~~~~~~~