[TASK] Document ability to use distinct flashmessage queues (#5439)

Author: garvinhicking

Documentation/ApiOverview/FlashMessages/Extbase.rst

@@ -39,8 +39,8 @@ A more elaborate example:
    );
 
 
-The messages are then displayed by Fluid with the relevant Viewhelper
-as shown in this excerpt of :file:`EXT:examples/Resources/Private/Layouts/Module.html`:
+The messages are then displayed by Fluid with the 
+`FlashMessages ViewHelper <f:flashMessages> <https://docs.typo3.org/permalink/t3viewhelper:typo3-fluid-flashmessages>`_:
 
 .. code-block:: html
 
@@ -53,3 +53,49 @@ as shown in this excerpt of :file:`EXT:examples/Resources/Private/Layouts/Module
 
 Where to display the flash messages in an Extbase-based backend module is
 as simple as moving the ViewHelper around.
+
+By default, all messages are put into the scope of the
+current plugin namespace with a prefix `extbase.flashmessages.`. So
+if your plugin namespace is computed as `tx_myvendor_myplugin`, the
+flash message queue identifier will be
+`extbase.flashmessages.tx_myvendor_myplugin`.
+
+..  _flash-messages-extbase-distinct:
+
+Using explicit flash message queues in Extbase
+==============================================
+
+It is possible to add a message to a different flash message queue. Use 
+cases could be a detailed display of different flash message queues in 
+different places of the page  or displaying a flash message when you 
+forward to a different controller or even a different extension. 
+
+If you need distinct queues, you can use a custom identifier to fetch
+and operate on that queue:
+
+..  code-block:: php
+
+    $customQueue = $this->getFlashMessageQueue('tx_myvendor_customqueue');
+    // Instead of using $this->addFlashMessage() you will instead directly
+    // access the custom queue:
+    $flashMessage = GeneralUtility::makeInstance(
+            FlashMessage::class,
+            'My flash message in a custom queue',
+            'My flash message title of a custom queue',
+            ContextualFeedbackSeverity::OK,
+            $storeInSession = true,
+    );
+    $customQueue->enqueue($flashMessage);
+
+..  _flash-messages-extbase-fluid-queueidentifier:
+
+Fluid flash messages ViewHelper with explicit queue identifier
+==============================================================
+
+When you used an :ref:`explicit flash message queue <flash-messages-extbase-distinct>`
+during enqueueing the message, it will only be displayed on the page if you use the
+same identifier in the `FlashMessages ViewHelper <f:flashMessages> <https://docs.typo3.org/permalink/t3viewhelper:typo3-fluid-flashmessages>`_.
+
+..  code-block:: html
+
+    <f:flashMessages queueIdentifier="tx_myvendor_customqueue" />