Update UIElement.xml (#9514)

Author: hadicpcp

xml/System.Windows/UIElement.xml

@@ -2038,9 +2038,7 @@ Image with an Elliptical Clip Region
 
  If the related properties are not already `true`, when you call  <xref:System.Windows.UIElement.Focus%2A>, one or more of the following events are raised in the following order: <xref:System.Windows.UIElement.PreviewLostKeyboardFocus>, <xref:System.Windows.UIElement.PreviewGotKeyboardFocus> (source is the new focus target), <xref:System.Windows.UIElement.IsKeyboardFocusedChanged>, <xref:System.Windows.UIElement.IsKeyboardFocusWithinChanged>, <xref:System.Windows.UIElement.LostKeyboardFocus>, <xref:System.Windows.UIElement.GotKeyboardFocus> (source is the new focus target).  
 
- In order for this call to be successful, some other element in the application needed to have focus previously.  
-  
-   
+For this call to be successful, some other element in the application needed to have focus previously. Also, when a parent container element calls this method, in certain situations the child gets the focus and the return value will be `false`. To test if the element has focus, use the `IsKeyboardFocusWithin` and `IsKeyboardFocused` properties.
 
 ## Examples  
  The following example sets focus to a <xref:System.Windows.Controls.TextBox> referenced by <xref:System.Windows.FrameworkElement.Name%2A>, and then adjusts the position of the cursor within the <xref:System.Windows.Controls.TextBox>.