minor #18579 [Config] Update definition.rst (JuGid)

This PR was submitted for the 6.3 branch but it was merged into the 5.4 branch instead.

Discussion
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[Config] Update definition.rst

Always acting as a trap for me : the configuration processor consider the root node (TreeBuilder "name") already stripped off. Always wanted to add documentation about it and never found out where to find this information.

Commits
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847c1a0 Update definition.rst

Author: javiereguiluz

components/config/definition.rst

@@ -870,3 +870,9 @@ Otherwise the result is a clean array of configuration values::
         $databaseConfiguration,
         $configs
     );
+
+.. caution::
+
+    When processing the configuration tree, the processor assumes the top level 
+    array key is already stripped off. If you want a root name, you should
+    add it to your tree builder as an array node.