Merge pull request ceph#15838 from theanalyst/doc-pool-ns-update

doc: update the rados namespace docs

Reviewed-by: Casey Bodley <cbodley@redhat.com>

doc/rados/operations/user-management.rst

@@ -228,10 +228,10 @@ namespace such that reads and writes by the user take place only within the
 namespace. Objects written to a namespace within the pool can only be accessed
 by users who have access to the namespace.
 
-.. note:: Currently (i.e., ``firefly``), namespaces are only useful for 
-   applications written on top of ``librados``. Ceph clients such as block 
-   device, object storage and file system do not currently support this 
-   feature.
+.. note:: Namespaces are primarily useful for applications written on top of
+   ``librados`` where the logical grouping can alleviate the need to create
+   different pools. Ceph Object Gateway (from ``luminous``) uses namespaces for various
+   metadata objects.
 
 The rationale for namespaces is that pools can be a computationally expensive
 method of segregating data sets for the purposes of authorizing separate sets