Add documentation for addFlag negatable option

pkg/args/args.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.
 
 /**
  * This library lets you define parsers for parsing raw command-line arguments
  * into a set of options and values using [GNU][] and [POSIX][] style options.
  *
  * ## Defining options ##
  *
  * To use this library, you create an [ArgParser] object which will contain
  * the set of options you support:
  *
  *     var parser = new ArgParser();
  *
  * Then you define a set of options on that parser using [addOption()] and
  * [addFlag()]. The minimal way to create an option is:
  *
  *     parser.addOption('name');
  *
  * This creates an option named "name". Options must be given a value on the
  * command line. If you have a simple on/off flag, you can instead use:
  *
  *     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".)
  *
  * Options may have an optional single-character abbreviation:
  *
  *     parser.addOption('mode', abbr: 'm');
  *     parser.addFlag('verbose', abbr: 'v');
  *
  * They may also specify a default value. The default value will be used if the
  * option isn't provided:
@@ skipping 729 matching lines @@
       allowedBuffer.add(allowed);
       if (allowed == option.defaultValue) {
         allowedBuffer.add(' (default)');
       }
       first = false;
     }
     allowedBuffer.add(']');
     return allowedBuffer.toString();
   }
 }