add library descriptions for sequence_zip and the libraries it exports

pkg/sequence_zip/lib/iterable_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 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";
 
 /**
  * Iterable that iterates over lists of values from other iterables.
  *
  * When [iterator] is read, an [Iterator] is created for each [Iterable] in
  * the [Iterable] passed to the constructor.
  *
@@ skipping 32 matching lines @@
     }
     _current = new List(_iterators.length);
     for (int i = 0; i < _iterators.length; i++) {
       _current[i] = _iterators[i].current;
     }
     return true;
   }
 
   List get current => _current;
 }