[commands] Document that cog_command_error must be async

discord/ext/commands/cog.py

@@ -312,14 +312,14 @@ class Cog(metaclass=CogMeta):
         return True
 
     @_cog_special_method
-    def cog_command_error(self, ctx, error):
+    async def cog_command_error(self, ctx, error):
         """A special method that is called whenever an error
         is dispatched inside this cog.
 
         This is similar to :func:`.on_command_error` except only applying
         to the commands inside this cog.
 
-        This function **can** be a coroutine.
+        This **must** be a coroutine.
 
         Parameters
         -----------