Add documentation

Document the behaviour and interaction between exec_config and
run_task_kwargs config

docs/apache-airflow-providers-amazon/executors/ecs-executor.rst

@@ -113,6 +113,9 @@ For a more detailed description of available options, including type
 hints and examples, see the ``config_templates`` folder in the Amazon
 provider package.
 
+.. note::
+   ``exec_config`` is an optional parameter that can be provided to operators. It is a dictionary type and in the context of the ECS Executor it represents a ``run_task_kwargs`` configuration which is then updated over-top of the ``run_task_kwargs`` specified in Airflow config above (if present). It is a recursive update which essentially applies Python update to each nested dictionary in the configuration. Loosely approximated as: ``run_task_kwargs.update(exec_config)``
+
 .. _dockerfile_for_ecs_executor:
 
 Dockerfile for ECS Executor