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

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 57bf59e4f6158dd98b252f357dd00309db20c646..b35a5ad5008902fc37717d11a5b37e52cccd899b 100644
--- a/sdk/lib/core/num.dart
+++ b/sdk/lib/core/num.dart
@@ -6,6 +6,9 @@ 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. */