doc: clarify use of location: in host spec

It wasn't clear that you can specify more than one element of the CRUSH hierarchy in a spec file, nor that it might be useful to do so (e.g. to ensure the host ends up beneath the default root).

So update the text to make it clearer, and similarly the example.

Signed-off-by: Matthew Vernon <[email protected]>

Author: mcv21

doc/cephadm/host-management.rst

@@ -303,14 +303,18 @@ Setting the initial CRUSH location of host
 ==========================================
 
 Hosts can contain a ``location`` identifier which will instruct cephadm to 
-create a new CRUSH host located in the specified hierarchy.
+create a new CRUSH host bucket located in the specified hierarchy.
+You can specify more than one element of the tree when doing so (for
+instance if you want to ensure that the rack that a host is being
+added to is also added to the default bucket), for example:
 
 .. code-block:: yaml
 
     service_type: host
     hostname: node-00
     addr: 192.168.0.10
     location:
+      root: default
       rack: rack1
 
 .. note::