Update nested param documentation

close #1625

Author: sloria

src/marshmallow/fields.py

@@ -452,8 +452,7 @@ class ParentSchema(Schema):
         # No
         author = fields.Nested(UserSchema(), only=('id', 'name'))
 
-    :param nested: The Schema class or class name (string)
-        to nest, or ``"self"`` to nest the :class:`Schema` within itself.
+    :param nested: `Schema` instance, class, class name (string), or callable that returns a `Schema` instance.
     :param exclude: A list or tuple of fields to exclude.
     :param only: A list or tuple of fields to marshal. If `None`, all fields are marshalled.
         This parameter takes precedence over ``exclude``.