add missing docs (#3882)

Author: mairaw

xml/System.Windows.Navigation/BaseUriHelper.xml

@@ -32,7 +32,8 @@
         <AssemblyVersion>4.0.0.0</AssemblyVersion>
       </AssemblyInfo>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Gets or sets the base uniform resource identifier (URI).</summary>
+        <value>The base uniform resource identifier (URI).</value>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>