[Debug] Add resolve-env option documentation of debug:config command

Author: alexandre-daubois

reference/configuration/debug.rst

@@ -16,6 +16,12 @@ key in your application configuration.
     # displays the actual config values used by your application
     $ php bin/console debug:config debug
 
+.. note::
+
+    The ``--resolve-env`` option is available with the ``debug:config``
+    command, which will resolved environment variables when
+    displaying the configuration values.
+
 .. note::
 
     When using XML, you must use the ``http://symfony.com/schema/dic/debug``