Update doc

Author: WaVEV

docs/source/releases/5.1.x.rst

@@ -9,6 +9,8 @@ Django MongoDB Backend 5.1.x
 
 - Backward-incompatible: :class:`~django_mongodb_backend.fields.ArrayField`\'s
   ``size`` argument is renamed to ``max_size``.
+- Added the ``size`` parameter to :class:`~django_mongodb_backend.fields.ArrayField`
+  for enforcing fixed-length arrays.
 - Added support for :doc:`database caching </topics/cache>`.
 
 5.1.0 beta 1