Update documentation for num.clamp.
(partially) fixes #27923
BUG= http://dartbug.com/27923
R=lrn@google.com
Review URL: https://codereview.chromium.org/2541513002 .
diff --git a/sdk/lib/core/num.dart b/sdk/lib/core/num.dart
index 21f23ee..ae9df01 100644
--- a/sdk/lib/core/num.dart
+++ b/sdk/lib/core/num.dart
@@ -296,6 +296,9 @@
* 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);
