[admin_manual] Add documentaion for thread summary

Signed-off-by: hamza221 <hamzamahjoubi221@gmail.com>

admin_manual/groupware/mail.rst

@@ -81,3 +81,13 @@ Congratulations! You are now able to use hosted Microsoft Outlook accounts in th
 .. figure:: images/azure_xoauth2/9.png
 
 .. figure:: images/azure_xoauth2/10.png
+
+Thread Summary
+--------------
+The mail app supports summarizing message threads that contain 3 or more messages. 
+
+.. warning:: `A text generation AI integration <https://apps.nextcloud.com/apps/integration_openai>`_ should be already in place to enable this feature.
+
+The feature is opt-in, it is disabled by default and can be enabled in mail adminstration settings.
+
+Adminstration settings > Groupware > Mail app > Enable thread summary