[TASK] Update documentation

Add a hint for users of the new TYPO3 v9 Site Configuration
feature.

Documentation/Administration/Index.rst

@@ -17,8 +17,15 @@ Once installed the extension is already active. By default the 404 Page is a
 simple pure HTML template, that includes a title and a message with almost
 no styles.
 
-**Important:** If your default website language is not english, configure
-your default language code in the extension manager.
+.. hint::
+   As of TYPO3 version 9, the new Site Configuration is in place. In the 
+   `errorHandling` section, make sure, there are no handlers registered
+   for `errorCode` values 404 and 403. Because they would supersede this
+   extension.
+
+.. important::
+   If your default website language is not english, configure
+   your default language code in the extension manager.
 
 Suggestions
 ^^^^^^^^^^^