add library descriptions for sequence_zip and the libraries it exports

pkg/sequence_zip/lib/sequence_zip.dart

Index: pkg/sequence_zip/lib/sequence_zip.dart
diff --git a/pkg/sequence_zip/lib/sequence_zip.dart b/pkg/sequence_zip/lib/sequence_zip.dart
index e8331ac4adb7a6ef0b38443835b5c36fe8eba911..2f0d7e6c4fd02a48efb8c41b0888fc0f54699989 100644
--- a/pkg/sequence_zip/lib/sequence_zip.dart
+++ b/pkg/sequence_zip/lib/sequence_zip.dart
@@ -2,6 +2,15 @@
 // 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.
 
+/**
+ * Utilities for combining multiple streams or Iterables
+ * into a single stream or Iterable, respectively.
+ *
+ * This library defines no new APIs.
+ * It's a convenience library for using the APIs in the
+ * [iterable_zip](#iterable_zip) and
+ * [stream_zip](#stream_zip) libraries.
+ */
 library sequence_zip;
 
 export "iterable_zip.dart";