Add see also link (dotnet#7637)

Author: gewarren

xml/System.Windows.Controls.Primitives/TextBoxBase.xml

@@ -2840,6 +2840,7 @@ If this property is `null`, selected text is not rendered.
  ]]></format>
         </remarks>
         <altmember cref="T:System.Windows.Controls.TextChangedEventHandler" />
+        <related type="Article" href="/dotnet/desktop/wpf/controls/how-to-detect-when-text-in-a-textbox-has-changed">How to: Detect When Text in a TextBox Has Changed</related>
       </Docs>
     </Member>
     <Member MemberName="TextChangedEvent">