Update documentation for num.clamp.

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;
 
 /**
  * An integer or floating-point number.
  *
  * It is a compile-time error for any type other than [int] or [double]
@@ skipping 278 matching lines @@
    * double.
    */
   double truncateToDouble();
 
   /**
    * Returns this [num] clamped to be in the range [lowerLimit]-[upperLimit].
    *
    * The comparison is done using [compareTo] and therefore takes `-0.0` into
    * account. This also implies that [double.NAN] is treated as the maximal
    * double value.
+   *
+   * The arguments [lowerLimit] and [upperLimit] must form a valid range where
+   * `lowerLimit.compareTo(upperLimit) <= 0`.
    */
   num clamp(num lowerLimit, num upperLimit);
 
   /** Truncates this [num] to an integer and returns the result as an [int]. */
   int toInt();
 
   /**
    * Return this [num] as a [double].
    *
    * If the number is not representable as a [double], an
@@ skipping 135 matching lines @@
     result = double.parse(source, _returnDoubleNull);
     if (result != null) return result;
     if (onError == null) throw new FormatException(input);
     return onError(input);
   }
 
   /** Helper functions for [parse]. */
   static int _returnIntNull(String _) => null;
   static double _returnDoubleNull(String _) => null;
 }