Update ScrollViewer.xml (#4613)

Refer to ScrollableHeight and ScrollableWidth properties instead of the more complicated explanation in terms of a subtraction. I checked the reference sources and these properties internally perform that same subtraction.

Author: DavidHollman

xml/System.Windows.Controls/ScrollViewer.xml

@@ -865,7 +865,7 @@
 ## Remarks  
  A positive <xref:System.Windows.Controls.ScrollViewer.HorizontalOffset%2A> value corresponds to the content being offset to the left.  
   
- Valid values are between zero and the <xref:System.Windows.Controls.ScrollViewer.ExtentWidth%2A> minus the <xref:System.Windows.Controls.ScrollViewer.ViewportWidth%2A>.  
+ Valid values are between zero and <xref:System.Windows.Controls.ScrollViewer.ScrollableWidth%2A>.  
   
  The returned value is described in Device Independent Pixels.  
   
@@ -2626,7 +2626,7 @@
 ## Remarks  
  A positive <xref:System.Windows.Controls.ScrollViewer.VerticalOffset%2A> value corresponds to the content being offset to the top.  
   
- Valid values are between zero and the <xref:System.Windows.Controls.ScrollViewer.ExtentHeight%2A> minus the <xref:System.Windows.Controls.ScrollViewer.ViewportHeight%2A>.  
+ Valid values are between zero and <xref:System.Windows.Controls.ScrollViewer.ScrollableHeight%2A>.  
   
  If <xref:System.Windows.Controls.ScrollViewer.CanContentScroll%2A> is `true`, the values of the <xref:System.Windows.Controls.ScrollViewer.ExtentHeight%2A>, <xref:System.Windows.Controls.ScrollViewer.ScrollableHeight%2A>, <xref:System.Windows.Controls.ScrollViewer.ViewportHeight%2A>, and <xref:System.Windows.Controls.ScrollViewer.VerticalOffset%2A> properties are number of items. If <xref:System.Windows.Controls.ScrollViewer.CanContentScroll%2A> is `false`, the values of these properties are Device Independent Pixels.