[AIRFLOW-XXX] Add Hint at user defined macros (#4885)

docs/code.rst

@@ -327,8 +327,10 @@ Sensors
 
 Macros
 ------
-Here's a list of variables and macros that can be used in templates
+Variables and macros can be used in templates (see the :ref:`jinja-templating` section)
 
+The following come for free out of the box with Airflow. 
+Additional custom macros can be added globally through :ref:`plugins`, or at a DAG level through the ``DAG.user_defined_macros`` argument.
 
 Default Variables
 '''''''''''''''''