add library descriptions for sequence_zip and the libraries it exports

pkg/sequence_zip/lib/stream_zip.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+/**
+ * Help for combining multiple streams into a single stream.
+ *
+ * This API is also available as part of the
+ * [sequence_zip](#sequence_zip) library.
+ */
 library stream_zip;
 
 import "dart:async";
 
 /**
  * A stream that combines the values of other streams.
  */
 class StreamZip extends Stream<List> {
   final Iterable<Stream> _streams;
   StreamZip(Iterable<Stream> streams) : _streams = streams;
@@ skipping 92 matching lines @@
 
     if (subscriptions.isEmpty) {
       controller.close();
     }
     return controller.stream.listen(onData,
                                     onError: onError,
                                     onDone: onDone,
                                     cancelOnError: cancelOnError);
   }
 }