core: getProfileName no raise ProfileUnknownError + minor doc fixes

src/memory/disco.py

@@ -154,7 +154,8 @@ class Discovery(object):
         @param type_: identitiy's type
         @param jid_: the jid of the target server (None for profile's server)
         @param profile_key: %(doc_profile_key)s
-        @return: a set of entities or None if no cached data were found
+        @return: a set of found entities
+        @raise CancelError: the request timed out
         """
         found_entities = set()