Added obsolete note to type summary (#2483)

Author: Ron Petrusha

xml/System.Net.Mail/SmtpClient.xml

@@ -35,7 +35,7 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>Allows applications to send email by using the Simple Mail Transfer Protocol (SMTP).</summary>
+    <summary>Allows applications to send email by using the Simple Mail Transfer Protocol (SMTP). The <c>SmtpClient</c> type is now obsolete.</summary>
     <remarks>
       <format type="text/markdown"><![CDATA[