Improve DartDocs for dart:core classes that may not be extended or implemented.

sdk/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.
 
 part of dart.core;
 
 /**
  * All numbers in dart are instances of [num].
+ *
+ * It is a compile-time error for any type other than the types int and double
+ * to attempt to extend or implement num.
  */
 abstract class num implements Comparable<num> {
   /** Addition operator. */
   num operator +(num other);
 
   /** Subtraction operator. */
   num operator -(num other);
 
   /** Multiplication operator. */
   num operator *(num other);
@@ skipping 246 matching lines @@
    *
    * Note: the conversion may round the output if the returned string
    * is accurate enough to uniquely identify the input-number.
    * For example the most precise representation of the [double] `9e59` equals
    * `"899999999999999918767229449717619953810131273674690656206848"`, but
    * this method returns the shorter (but still uniquely identifying) `"9e59"`.
    *
    */
   String toString();
 }