Add allow_null serialization output note (#5641)

Author: Ryan P Kilby

docs/api-guide/fields.md

@@ -45,6 +45,8 @@ Defaults to `True`.
 
 Normally an error will be raised if `None` is passed to a serializer field. Set this keyword argument to `True` if `None` should be considered a valid value.
 
+Note that setting this argument to `True` will imply a default value of `null` for serialization output, but does imply a default for input deserialization.
+
 Defaults to `False`
 
 ### `default`