[Excel] (Undo) Add Insiders note (#5207)

* [Excel] (Undo) Add Insiders note

* Adjust phrasing

Author: alison-mk

docs/excel/excel-add-ins-undo-capabilities.md

@@ -1,7 +1,7 @@
 ---
 title: Undo capabilities with the Excel JavaScript API
 description: Learn how to preserve the undo stack in your Excel add-ins.
-ms.date: 05/15/2025
+ms.date: 06/04/2025
 ms.localizationpriority: medium
 ---
 
@@ -10,7 +10,7 @@ ms.localizationpriority: medium
 Excel add-ins support undo behavior. This preserves both actions performed by Excel JavaScript APIs and actions performed by the user in Excel. These actions are saved in the *undo stack* for an individual user, allowing the user to step back through their actions when desired.
 
 > [!NOTE]
-> The features described in this article are currently available only in public preview. [!INCLUDE [Information about using preview APIs](../includes/using-excel-preview-apis.md)]
+> The features described in this article are currently available only in public preview. [!INCLUDE [Information about using preview APIs](../includes/using-excel-preview-apis.md)] To use this feature, you must also join the [Microsoft 365 Insider Program](https://aka.ms/MSFT365InsiderProgram) and download the latest build of Excel.
 
 ## Undo grouping