Merge branch '6.3' into 6.4

* 6.3:
  Update definition.rst

Author: javiereguiluz

components/config/definition.rst

@@ -893,3 +893,8 @@ Otherwise the result is a clean array of configuration values::
         $databaseConfiguration,
         $configs
     );
+
+.. caution::
+
+    When processing the configuration tree, the processor assumes that the top
+    level array key (which matches the extension name) is already stripped off.