Temporarily remove obsolete attribute (#3320)

* Temporarily remove obsolete attribute

* feedback

* expand note

* fix typo + improvements

Author: mairaw

xml/System.Net.Mail/SmtpClient.xml

@@ -29,23 +29,25 @@
       <InterfaceName>System.IDisposable</InterfaceName>
     </Interface>
   </Interfaces>
-  <Attributes>
-    <Attribute FrameworkAlternate="xamarinandroid-7.1;xamarinios-10.8;xamarinmac-3.0">
-      <AttributeName>System.Obsolete("SmtpClient and its network of types are poorly designed, we strongly recommend you use https://github.com/jstedfast/MailKit and https://github.com/jstedfast/MimeKit instead")</AttributeName>
-    </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>
     <remarks>
       <format type="text/markdown"><![CDATA[  
   
-## Remarks  
- The `System.Net.Mail.SmtpClient` class is used to send email to an SMTP server for delivery. The SMTP protocol is defined in RFC 2821, which is available at [https://www.ietf.org](https://www.ietf.org/).  
+## Remarks
+
+The `SmtpClient` class is used to send email to an SMTP server for delivery. The SMTP protocol is defined in RFC 2821, which is available at [https://www.ietf.org](https://www.ietf.org/).  
 
 > [!IMPORTANT]
->  We don't recommend that you use the `SmtpClient` class for new development. For more information, see [SmtpClient shouldn't be used](https://github.com/dotnet/platform-compat/blob/master/docs/DE0005.md) on GitHub.
+> We don't recommend that you use the `SmtpClient` class for new development because `SmtpClient` doesn't support many modern protocols. Use [MailKit](https://github.com/jstedfast/MailKit) or other libraries instead. For more information, see [SmtpClient shouldn't be used](https://github.com/dotnet/platform-compat/blob/master/docs/DE0005.md) on GitHub.
+
+The `SmtpClient` class is obsolete in Xamarin. However:
+
+- It is included in the .NET Standard 2.0 and later versions and therefore must be part of any .NET implementation that supports those versions.
+- It is present and can be used in .NET Framework 4 through .NET Framework 4.8.
+- It is usable in .NET Core, but its use isn't recommended.
 
- The classes shown in the following table are used to construct email messages that can be sent using <xref:System.Net.Mail.SmtpClient>.  
+The classes shown in the following table are used to construct email messages that can be sent using <xref:System.Net.Mail.SmtpClient>.  
   
 |Class|Description|  
 |-----------|-----------------|