Add documentation on bot_check_once

discord/ext/commands/cog.py

@@ -296,6 +296,10 @@ class Cog(metaclass=CogMeta):
         """A special method that registers as a :meth:`.Bot.check_once`
         check.
 
+        Note that when using this function, context sent from a group command
+        may only parse the parent command and not the subcommands due to it
+        being invoked once per :meth:`.Command.invoke` call.
+
         This function **can** be a coroutine and must take a sole parameter,
         ``ctx``, to represent the :class:`.Context`.
         """