Fix documentation for RoleConverter

Author: Jackenmen

discord/ext/commands/converter.py

@@ -608,8 +608,8 @@ async def convert(self, ctx, argument):
 class RoleConverter(IDConverter):
     """Converts to a :class:`~discord.Role`.
 
-    All lookups are via the local guild. If in a DM context, then the lookup
-    is done by the global cache.
+    All lookups are via the local guild. If in a DM context, the converter raises
+    :exc:`.NoPrivateMessage` exception.
 
     The lookup strategy is as follows (in order):