Add comments for associate/dissociate logic

bin/nova-manage

@@ -817,6 +817,10 @@ class NetworkCommands(object):
         admin_context = context.get_admin_context()
         network = db.network_get_by_cidr(admin_context, fixed_range)
         net = {}
+        #User can choose the following actions each for project and host.
+        #1) Associate (set not None value given by project/host parameter)
+        #2) Disassociate (set None by disassociate parameter)
+        #3) Keep unchanged (project/host key is not added to 'net')
         if project:
             net['project_id'] = project
         elif dis_project: