add library descriptions for sequence_zip and the libraries it exports

pkg/sequence_zip/lib/iterable_zip.dart

Index: pkg/sequence_zip/lib/iterable_zip.dart
diff --git a/pkg/sequence_zip/lib/iterable_zip.dart b/pkg/sequence_zip/lib/iterable_zip.dart
index 90c4c4ff5dce64e33b08461c3cd4c1c8dd04c0da..f21da73960f40b350c6905c74c51ae6de9c926d2 100644
--- a/pkg/sequence_zip/lib/iterable_zip.dart
+++ b/pkg/sequence_zip/lib/iterable_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 Iterables into a single Iterable.
+ *
+ * This API is also available as part of the
+ * [sequence_zip](#sequence_zip) library.

[Lasse Reichstein Nielsen, 2013/08/26 13:57:30]

Will the fragment-link work in all cases where this documentation is shown?
E.g., at the top of the page for this library?

[Kathy Walrath, 2013/08/26 14:15:58]

Good question. It works in the new dartdoc, but almost certainly won't in the
old dartdoc (even once those libraries are included in the generated output).

We have this problem in a few other docs, too. I guess I'm inclined to leave the
links in, but to open a bug about this difference between the two doc
generators. I'll check with Emily.

+ */
 library iterable_zip;
 
 import "dart:collection";