add library descriptions for sequence_zip and the libraries it exports

pkg/sequence_zip/lib/stream_zip.dart

Index: pkg/sequence_zip/lib/stream_zip.dart
diff --git a/pkg/sequence_zip/lib/stream_zip.dart b/pkg/sequence_zip/lib/stream_zip.dart
index d1a47ab7a504441dddaeae9f2a6953d92b1b3461..0b4b40ad9cfb2778b39e036066ffb0f925a8b9ce 100644
--- a/pkg/sequence_zip/lib/stream_zip.dart
+++ b/pkg/sequence_zip/lib/stream_zip.dart
@@ -2,6 +2,12 @@
 // 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";