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

sdk/lib/core/double.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;
 
 // TODO: Convert this abstract class into a concrete class double
 // that uses the patch class functionality to account for the
 // different platform implementations.
 
 /**
  * Representation of Dart doubles containing double specific constants
  * and operations and specializations of operations inherited from
- * [num].
+ * [num]. Dart doubles are 64-bit floating-point numbers as specified in the
+ * IEEE 754 standard.
  *
  * The [double] type is contagious. Operations on [double]s return
  * [double] results.
+ *
+ * It is a compile-time error for a class to attempt to extend or implement
+ * double.
  */
 abstract class double extends num {
   static const double NAN = 0.0 / 0.0;
   static const double INFINITY = 1.0 / 0.0;
   static const double NEGATIVE_INFINITY = -INFINITY;
   static const double MIN_POSITIVE = 5e-324;
   static const double MAX_FINITE = 1.7976931348623157e+308;
 
   double remainder(num other);
 
@@ skipping 131 matching lines @@
    *     "0."
    *     ".0"
    *     "-1.e3"
    *     "1234E+7"
    *     "+.12e-9"
    *     "-NaN"
    */
   external static double parse(String source,
                                [double handleError(String source)]);
 }