Merge pull request #2463 from citelao/patch-4

[FocusManager] Document that some API only works for UWP

Author: Karl-Bridge-Microsoft

windows.ui.xaml.input/focusmanager_getfocusedelement_1183614552.md

@@ -22,6 +22,12 @@ The object that has focus. Typically, this is a [Control](../windows.ui.xaml.con
 
 ## -remarks
 
+> [!WARNING]
+> For WinUI 3 apps, this method always returns `null` as it relies on CoreWindow APIs not supported by WinUI 3.
+>
+> Instead, use the [FocusManager.GetFocusedElement(XamlRoot)](./windows.ui.xaml.input.focusmanager.getfocusedelement#windows-ui-xaml-input-focusmanager-getfocusedelement(windows-ui-xaml-xamlroot))
+> overload, where `XamlRoot` can be retrieved any loaded UIElement through the [XamlRoot](/uwp/api/windows.ui.xaml.uielement.xamlroot) property.
+
 ## -examples
 
 ## -see-also