fix NetworkStream reading example description (#10898)

Remove incorrect DataAvailable reference and clarify buffer usage

Author: flafarga

xml/System.Net.Sockets/NetworkStream.xml

@@ -1562,8 +1562,7 @@ There is a failure reading from the network.</exception>
 
 
 ## Examples
- The following code example uses <xref:System.Net.Sockets.NetworkStream.DataAvailable%2A> to determine if data is available to be read. If data is available, it reads from the <xref:System.Net.Sockets.NetworkStream>.
-
+ The following code example reads data from the <xref:System.Net.Sockets.NetworkStream> into a buffer. If data is available, it continues reading until the end of the stream is reached, appending the received data to a <xref:System.Text.StringBuilder>.
  :::code language="csharp" source="~/snippets/csharp/System.Net.Sockets/NetworkStream/Overview/source.cs" id="Snippet4":::
 
  ]]></format>