Add documentation for addFlag negatable option

pkg/args/args.dart

Index: pkg/args/args.dart
diff --git a/pkg/args/args.dart b/pkg/args/args.dart
index acc557aed722942b6141e16f18766c1518f80002..ad6688a2e19e5fc019b8ea65ff59e551cd1910be 100644
--- a/pkg/args/args.dart
+++ b/pkg/args/args.dart
@@ -23,6 +23,11 @@
  *
  *     parser.addFlag('name');
  *
+ * Flag options will, by default, accept a 'no-' prefix to negate the option. 
+ * This can be disabled as such:
+ * 
+ *     paser.addFlag('name', negatable: false);
+ * 
  * (From here on out "option" will refer to both "regular" options and flags.
  * In cases where the distinction matters, we'll use "non-flag option".)
  *