Closes #15315 by documenting the SlackOptions::threadTs() method

Author: 0x346e3730

_images/notifier/slack/message-reply.png

@@ -131,10 +131,41 @@ The result will be something like:
 .. image:: /_images/notifier/slack/field-method.png
    :align: center
 
-.. versionadded:: 5.1
+.. versionadded:: 5.3
 
     The `field()` method was introduced in Symfony 5.1.
 
+Sending a Slack Message as a reply
+-------------------------------------------
+
+To send your slack message as a reply in a thread you can use the
+:method:`SlackOptions::threadTs() <Symfony\\Component\\Notifier\\Bridge\\Slack\\SlackOptions::threadTs>` method::
+
+    use Symfony\Component\Notifier\Bridge\Slack\Block\SlackSectionBlock;
+    use Symfony\Component\Notifier\Bridge\Slack\SlackOptions;
+    use Symfony\Component\Notifier\Message\ChatMessage;
+
+    $chatMessage = new ChatMessage('Symfony Feature');
+
+    $options = (new SlackOptions())
+        ->block((new SlackSectionBlock())->text('My reply'))
+        ->threadTs('1621592155.003100')
+    ;
+
+    // Add the custom options to the chat message and send the message
+    $chatMessage->options($options);
+
+    $chatter->send($chatMessage);
+
+The result will be something like:
+
+.. image:: /_images/notifier/slack/message-reply.png
+   :align: center
+
+.. versionadded:: 5.3
+
+    The `threadTs()` method was introduced in Symfony 5.3.
+
 Adding Interactions to a Discord Message
 ----------------------------------------