[Form] Added AbstractChoiceLoader

reference/forms/types/options/choice_loader.rst.inc

@@ -26,6 +26,22 @@ This will cause the call of ``StaticClass::getConstants()`` to not happen if the
 request is redirected and if there is no pre set or submitted data. Otherwise
 the choice options would need to be resolved thus triggering the callback.
 
+If the ``CallbackChoiceLoader`` doesn't fit your needs and you want to create
+your own loader, you can either create a loader by implementing the
+:class:`Symfony\\Component\\Form\\ChoiceList\\Loader\\ChoiceLoaderInterface`
+or by extending the
+:class:`Symfony\\Component\\Form\\ChoiceList\\Loader\\AbstractChoiceLoader`.
+This abstract class allows to save some boilerplate by implementing some
+of the :class:`Symfony\\Component\\Form\\ChoiceList\\Loader\\ChoiceLoaderInterface`
+methods. Thus, you'll only have to implement the
+:method:`Symfony\\Component\\Form\\ChoiceList\\Loader\\AbstractChoiceLoader::loadChoices`
+method to have a fully functional choice loader.
+
+.. versionadded:: 5.1
+
+    The :class:`Symfony\\Component\\Form\\ChoiceList\\Loader\\AbstractChoiceLoader`
+    class was introduced in Symfony 5.1.
+
 When you're defining a custom choice type that may be reused in many fields
 (like entries of a collection) or reused in multiple forms at once, you
 should use the :class:`Symfony\\Component\\Form\\ChoiceList\\ChoiceList`