Document merging behaviour of AllowedMentions in Messageable.send

discord/abc.py

@@ -827,7 +827,12 @@ class Messageable(metaclass=abc.ABCMeta):
             before deleting the message we just sent. If the deletion fails,
             then it is silently ignored.
         allowed_mentions: :class:`~discord.AllowedMentions`
-            Controls the mentions being processed in this message.
+            Controls the mentions being processed in this message. If this is
+            passed, then the object is merged with :attr:`~discord.Client.allowed_mentions`.
+            The merging behaviour only overrides attributes that have been explicitly passed
+            to the object, otherwise it uses the attributes set in :attr:`~discord.Client.allowed_mentions`.
+            If no object is passed at all then the defaults given by :attr:`~discord.Client.allowed_mentions`
+            are used instead.
 
             .. versionadded:: 1.4