Update documentation for num.clamp.

sdk/lib/core/num.dart

Index: sdk/lib/core/num.dart
diff --git a/sdk/lib/core/num.dart b/sdk/lib/core/num.dart
index 21f23eef18321ab7fc4b59aa397cdbb7ead0b0d6..ae9df01afc01f56caa0266efb0951e0e9479b1e7 100644
--- a/sdk/lib/core/num.dart
+++ b/sdk/lib/core/num.dart
@@ -296,6 +296,9 @@ abstract class num implements Comparable<num> {
    * 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);