Improving help text for RPC opt

Adding improved help text for allowed_rpc_exception_modules opt.

Change-Id: I4bf2257cef7d99110e67aefa58dad9055e4dfccb
Partial-Bug: #1570946

glance/common/rpc.py

@@ -36,15 +36,30 @@ LOG = logging.getLogger(__name__)
 
 
 rpc_opts = [
-    # NOTE(flaper87): Shamelessly copied
-    # from oslo rpc.
     cfg.ListOpt('allowed_rpc_exception_modules',
                 default=['glance.common.exception',
                          'builtins',
                          'exceptions',
                          ],
-                help='Modules of exceptions that are permitted to be recreated'
-                     ' upon receiving exception data from an rpc call.'),
+                help=_("""
+List of allowed exception modules to handle RPC exceptions.
+
+Provide a comma separated list of modules whose exceptions are
+permitted to be recreated upon receiving exception data via an RPC
+call made to Glance. The default list includes
+``glance.common.exception``, ``builtins``, and ``exceptions``.
+
+The RPC protocol permits interaction with Glance via calls across a
+network or within the same system. Including a list of exception
+namespaces with this option enables RPC to propagate the exceptions
+back to the users.
+
+Possible values:
+    * A comma separated list of valid exception modules
+
+Related options:
+    * None
+""")),
 ]
 
 CONF = cfg.CONF