Add doc for Clone CG

In Liberty, we introduced support for creating a
consistency group from a source consistency group.
This patch adds document for it. This patch also
adds a missing entry in the policy file for update
CG.

Cinder server side change is merged:
https://review.openstack.org/#/c/203473/

Cinder client side change is merged:
https://review.openstack.org/#/c/205897/

Change-Id: I4e9a2b42f1a55c2351c9b78259a71593e0ac21e5

doc/admin-guide-cloud/source/blockstorage-consistency-groups.rst

@@ -34,6 +34,7 @@ Here are existing policy entries for consistency groups::
 
     "consistencygroup:create": "group:nobody",
     "consistencygroup:delete": "group:nobody",
+    "consistencygroup:update": "group:nobody",
     "consistencygroup:get": "group:nobody",
     "consistencygroup:get_all": "group:nobody",
     "consistencygroup:create_cgsnapshot" : "group:nobody",
@@ -93,6 +94,8 @@ The following consistency group operations are supported:
 -  Create a consistency group from the snapshot of another consistency
    group.
 
+-  Create a consistency group from a source consistency group.
+
 The following operations are not allowed if a volume is in a consistency
 group:
 
@@ -295,3 +298,16 @@ consistency group::
 
     $ cinder consisgroup-create-from-src --cgsnapshot 6d9dfb7d-079a-471e-b75a-\
       6e9185ba0c38 --name 'new cg' --description 'new cg from cgsnapshot'
+
+**Create a consistency group from a source consistency group**::
+
+    $ cinder consisgroup-create-from-src
+    [--source-cg SOURCECG]
+    [--name NAME]
+    [--description DESCRIPTION]
+
+The parameter ``SOURCECG`` is a name or UUID of a source
+consistency group::
+
+    $ cinder consisgroup-create-from-src --source-cg 6d9dfb7d-079a-471e-b75a-\
+      6e9185ba0c38 --name 'new cg' --description 'new cloned cg'