Deprecated the SYMFONY__ variables

Author: javiereguiluz

configuration/external_parameters.rst

@@ -114,6 +114,12 @@ of the following:
 
 .. tip::
 
+    .. versionadded:: 3.3
+        The support of the special ``SYMFONY__`` environment variables was
+        deprecated in Symfony 3.3 and it will be removed in 4.0. Instead of
+        using those variables, define regular environment variables and get
+        their values using the ``%env(...)%`` syntax in your config files.
+
     You can also define the default value of any existing parameters using
     special environment variables named after their corresponding parameter
     prefixed with ``SYMFONY__`` after replacing dots by double underscores