Add note about Variable precedence with env vars (#21568)

This PR updates some documentation regarding setting Airflow Variables using environment variables. Environment variables take precedence over variables defined in the UI/metastore based on this default search path list: https://github.dev/apache/airflow/blob/7864693e43c40fd8f0914c05f7e196a007d16d50/airflow/secrets/__init__.py#L29-L30

docs/apache-airflow/howto/variable.rst

@@ -62,7 +62,8 @@ You can use them in your DAGs as:
     Single underscores surround ``VAR``.  This is in contrast with the way ``airflow.cfg``
     parameters are stored, where double underscores surround the config section name.
     Variables set using Environment Variables would not appear in the Airflow UI but you will
-    be able to use them in your DAG file.
+    be able to use them in your DAG file. Variables set using Environment Variables will also
+    take precedence over variables defined in the Airflow UI.
 
 Securing Variables
 ------------------