Add SmtpClient.SendMailAsync cancellation doc (#4749)

* SmtpClient.SendMailAsync doc

* Apply suggestions from code review

Co-authored-by: Carlos Sanchez <[email protected]>

Co-authored-by: Jan Jahoda <[email protected]>
Co-authored-by: Carlos Sanchez <[email protected]>

Author: 3 people

xml/System.Net.Mail/SmtpClient.xml

@@ -1640,11 +1640,20 @@ The classes shown in the following table are used to construct email messages th
         <Parameter Name="cancellationToken" Type="System.Threading.CancellationToken" Index="1" FrameworkAlternate="net-5.0" />
       </Parameters>
       <Docs>
-        <param name="message">To be added.</param>
-        <param name="cancellationToken">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
-        <remarks>To be added.</remarks>
+        <param name="message">The message to send.</param>
+        <param name="cancellationToken">The cancellation token to cancel the operation.</param>
+        <summary>Sends the specified message to an SMTP server for delivery as an asynchronous operation.</summary>
+        <returns>The task object representing the asynchronous operation.</returns>
+        <remarks>
+          <format type="text/markdown"><![CDATA[  
+  
+## Remarks  
+ This operation will not block. The returned <xref:System.Threading.Tasks.Task> object will complete once the message has been sent.  
+  
+ ]]></format>
+        </remarks>
+        <exception cref="T:System.ArgumentNullException">
+          <paramref name="message" /> is <see langword="null" />.</exception>
       </Docs>
     </Member>
     <Member MemberName="SendMailAsync">
@@ -1737,14 +1746,33 @@ The classes shown in the following table are used to construct email messages th
         <Parameter Name="cancellationToken" Type="System.Threading.CancellationToken" Index="4" FrameworkAlternate="net-5.0" />
       </Parameters>
       <Docs>
-        <param name="from">To be added.</param>
-        <param name="recipients">To be added.</param>
-        <param name="subject">To be added.</param>
-        <param name="body">To be added.</param>
-        <param name="cancellationToken">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
-        <remarks>To be added.</remarks>
+        <param name="from">The address information of the message sender.</param>
+        <param name="recipients">The addresses that the message is sent to.</param>
+        <param name="subject">The subject line for the message.</param>
+        <param name="body">The message body.</param>
+        <param name="cancellationToken">The cancellation token to cancel the operation.</param>
+        <summary>Sends the specified message to an SMTP server for delivery as an asynchronous operation, using the specified sender, recipients, subject, and body strings.</summary>
+        <returns>The task object representing the asynchronous operation.</returns>
+        <remarks>
+          <format type="text/markdown"><![CDATA[  
+  
+## Remarks  
+ This operation will not block. The returned <xref:System.Threading.Tasks.Task> object will complete once the message has been sent.  
+  
+ ]]></format>
+        </remarks>
+        <exception cref="T:System.ArgumentNullException">
+          <paramref name="from" /> is <see langword="null" />.  
+  
+ -or-  
+  
+ <paramref name="recipients" /> is <see langword="null" />.</exception>
+        <exception cref="T:System.ArgumentException">
+          <paramref name="from" /> is <see cref="F:System.String.Empty" />.  
+  
+ -or-  
+  
+ <paramref name="recipients" /> is <see cref="F:System.String.Empty" />.</exception>
       </Docs>
     </Member>
     <Member MemberName="ServicePoint">