Add documentation for NonDisposingStream class

Co-authored-by: adamhathcock <527620+adamhathcock@users.noreply.github.com>

src/SharpCompress/Writers/Zip/ZipWriter.cs

@@ -807,6 +807,8 @@ public class ZipWriter : AbstractWriter
 
     /// <summary>
     /// A stream wrapper that doesn't dispose the underlying stream when disposed.
+    /// This is used in encryption scenarios where the crypto stream would otherwise
+    /// dispose the counting stream prematurely, before we can read the final count.
     /// </summary>
     private class NonDisposingStream : Stream
     {