Update isolated-web-parts.md

Added a note that isolated web parts are not supported on Classic pages

Author: lucabandMSFT

docs/spfx/web-parts/isolated-web-parts.md

@@ -20,6 +20,7 @@ Isolated web parts introduce a new way to isolate access to APIs secured with Az
 
 > [!IMPORTANT]
 > The isolated web parts capability is available only in SharePoint Framework v1.8 and later.
+> The isolated web parts are only supported on modern pages. While you might be able to add isolated web parts to classic pages, upon publishing, the web part will not render and generate an error.
 
 ![Architectural overview illustrating how isolated web parts work](../../images/isolated-web-parts.png)