change default module description

Modules by default were returning None from help_description(), which
meant that they would not display in !help

Instead, return a generic not-available message. If a module really
doesn't want to display in !help, override and return None

Module.py

@@ -158,7 +158,7 @@ class Module(object):
         """Return a quick list of commands or other summary, should be
         less than two lines. If you want the module hidden from "!help",
         return None here"""
-        return None
+        return "No description available"
 
     def help_summary(self):
         """Return a command summary or longer description of this module.