Remark for null in UWP (#3771)

* Remark for null in UWP

Add a remark that in a UWP app you must use String.Empty rather than null for the property name (to indicate a reset).

* Update xml/System.ComponentModel/INotifyPropertyChanged.xml

Co-Authored-By: Maira Wenzel <[email protected]>

Co-authored-by: Maira Wenzel <[email protected]>

Author: MikeHillberg

xml/System.ComponentModel/INotifyPropertyChanged.xml

@@ -390,7 +390,7 @@ End Class
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- The <xref:System.ComponentModel.INotifyPropertyChanged.PropertyChanged> event can indicate all properties on the object have changed by using either `null` or <xref:System.String.Empty?displayProperty=nameWithType> as the property name in the <xref:System.ComponentModel.PropertyChangedEventArgs>.  
+ The <xref:System.ComponentModel.INotifyPropertyChanged.PropertyChanged> event can indicate all properties on the object have changed by using either `null` or <xref:System.String.Empty?displayProperty=nameWithType> as the property name in the <xref:System.ComponentModel.PropertyChangedEventArgs>. Note that in a UWP application, <xref:System.String.Empty?displayProperty=nameWithType> must be used rather than `null`.