docs: Document the prepare_osd tag

There are times where being able to skip OSD creation is useful to the
admin (see #1777 for example), and skipping the prepare_osd tag is a
way to achieve this. Document this fact.

Signed-off-by: Matthew Vernon <mv3@sanger.ac.uk>

docs/source/osds/scenarios.rst

@@ -21,6 +21,11 @@ The ``lvm`` scenario mentioned above support both containerized and non-containe
 As a reminder, deploying a containerized cluster can be done by setting ``containerized_deployment``
 to ``True``.
 
+If you want to skip OSD creation during a ``ceph-ansible run``
+(e.g. because you have already provisioned your OSDs but disk IDs have
+changed), you can skip the ``prepare_osd`` tag i.e. by specifying
+``--skip-tags prepare_osd`` on the ``ansible-playbook`` command line.
+
 .. _osd_scenario_lvm:
 
 lvm