Update thickness remark (#2569)

You can specify the Thickness values in three ways, not two.
Also added description of the two-value case. The wording can probably improved however...

Author: JeroenOortwijn

xml/System.Windows/Thickness.xml

@@ -32,7 +32,7 @@
       <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- You can specify <xref:System.Windows.Thickness> values in two ways. If you pass four <xref:System.Double> values to the structure, they represent the <xref:System.Windows.Thickness.Left%2A>, <xref:System.Windows.Thickness.Top%2A>, <xref:System.Windows.Thickness.Right%2A> and <xref:System.Windows.Thickness.Bottom%2A> sides, respectively, of the bounding rectangle. You can also supply a single value, which applies a uniform value to all four sides of the bounding rectangle.  
+ You can specify <xref:System.Windows.Thickness> values in three ways. If you pass four <xref:System.Double> values to the structure, they represent the <xref:System.Windows.Thickness.Left%2A>, <xref:System.Windows.Thickness.Top%2A>, <xref:System.Windows.Thickness.Right%2A> and <xref:System.Windows.Thickness.Bottom%2A> sides, respectively, of the bounding rectangle. You can also supply two <xref:System.Double> values, the first value represents the Left and Right sides of the bounding rectangle and the second value represents the Top and Bottom sides of the bounding rectangle. You can also supply a single <xref:System.Double> value, which applies a uniform value to all four sides of the bounding rectangle.  
   
 <a name="xamlAttributeUsage_Thickness"></a>   
 ## XAML Attribute Usage  
@@ -534,4 +534,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>