Document when async HttpListener send completes (#8470)

* Document when async HttpListener send completes
Fixes #2068

* Code review feedback

Author: rzikm

xml/System.Net/HttpListener.xml

@@ -1246,8 +1246,8 @@ The following code example demonstrates calling the `Close` method:
  If this instance is already stopped, calling this method has no effect.  
   
  After you have stopped an <xref:System.Net.HttpListener> object, you can use the <xref:System.Net.HttpListener.Start%2A> method to restart it.  
-  
-   
+
+ This method does not block to finish sending responses for currently accepted connections.
   
 ## Examples  
  

xml/System.Net/HttpListenerResponse.xml

@@ -430,9 +430,12 @@
 ## Remarks  
  If you are sending body data with the response, you can use this method to send it as a <xref:System.Byte> array instead of writing the body data to the <xref:System.Net.HttpListenerResponse.OutputStream%2A> property and calling the <xref:System.Net.HttpListenerResponse.Close%2A> method.  
   
- This method closes the response stream and the <xref:System.Net.HttpListenerRequest> associated with the response.  
-  
-   
+ This method closes the response stream and the <xref:System.Net.HttpListenerRequest> associated with the response.
+
+ If `false` is specified as the `willBlock`, the method returns immediately and the connection is closed asynchronously once the data is sent.
+
+> [!NOTE]
+>  If closing asynchronously, it is not possible for the caller to determine when the data have been sent. In addition to that, disposing the <xref:System.Net.HttpListenerResponse>, <xref:System.Net.HttpListenerContext> or <xref:System.Net.HttpListener> may close the connection before all the data are sent.
   
 ## Examples  
  The following code example demonstrates calling this method.