NegotiateStream - update documentation (#4335)

* NegotiateStream - update documentation

* format code

* Update xml/System.Net.Security/NegotiateStream.xml

Co-authored-by: Genevieve Warren <gewarren@microsoft.com>

* Remove example

Co-authored-by: Jan Jahoda <jajahoda@.microsoft.com>
Co-authored-by: Genevieve Warren <gewarren@microsoft.com>

Author: 3 people

samples/snippets/csharp/VS_Snippets_Remoting/NclNegoSyncClient/CS/client.cs

@@ -48,10 +48,10 @@ public static void Main(String[] args)
                  // Encode the test data into a byte array.
                 byte[] message = System.Text.Encoding.UTF8.GetBytes("Hello from the client.");
                 authStream.Write(message, 0, message.Length);
-          authStream.Flush();
+                authStream.Flush();
                 Console.WriteLine("Sent {0} bytes.", message.Length);
-         }
-         // Close the client connection.
+            }
+            // Close the client connection.
             authStream.Close();
             Console.WriteLine("Client closed.");
     }

xml/System.Net.Security/NegotiateStream.xml

@@ -3073,10 +3073,17 @@ Authentication has not occurred.</exception>
         <Parameter Name="cancellationToken" Type="System.Threading.CancellationToken" Index="0" FrameworkAlternate="net-5.0;netcore-2.0;netcore-2.1;netcore-2.2;netcore-3.0;netcore-3.1;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="cancellationToken">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
-        <remarks>To be added.</remarks>
+        <param name="cancellationToken">The token to monitor for cancellation requests.</param>
+        <summary>Asynchronously writes any buffered data to the underlying device.</summary>
+        <returns>A task that represents the asynchronous flush operation.</returns>
+        <remarks>
+          <format type="text/markdown"><![CDATA[  
+  
+## Remarks  
+ This method invokes <xref:System.IO.Stream.FlushAsync%2A> on the underlying stream.  
+ 
+ ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="ImpersonationLevel">