Update obsoletion note in summary (#7359)

Author: gewarren

xml/System.Net.Mail/SmtpClient.xml

@@ -52,7 +52,7 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>Allows applications to send email by using the Simple Mail Transfer Protocol (SMTP). The <c>SmtpClient</c> type is now obsolete.</summary>
+    <summary>Allows applications to send email by using the Simple Mail Transfer Protocol (SMTP). The <see cref="T:System.Net.Mail.SmtpClient" /> type is obsolete on some platforms and not recommended on others; for more information, see the Remarks section.</summary>
     <remarks>
       <format type="text/markdown"><![CDATA[