Add documentation of double and int.

lib/core/num.dart

 // Copyright (c) 2012, 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.
 
 // Dart core library.
 
+/**
+ * All numbers in dart are instances of [num].
+ */
 abstract class num implements Comparable, Hashable {
   /** Addition operator. */
   num operator +(num other);
 
   /** Subtraction operator. */
   num operator -(num other);
 
   /** Multiplication operator. */
   num operator *(num other);
 
@@ skipping 93 matching lines @@
    *
    * The [num] in converted to an [int] using [toInt]. That [int] is
    * then converted to a string representation with the given
    * [radix]. In the string representation, lower-case letters are
    * used for digits above '9'.
    *
    * The [radix] argument must be an integer between 2 and 36.
    */
   String toRadixString(int radix);
 }