Update documentation for Platform.version

sdk/lib/io/platform.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.
 
 part of dart.io;
 
 /**
  * Information about the environment in which the current program is running.
  *
  * Platform provides information such as the operating system,
@@ skipping 155 matching lines @@
    * Returns the value of the --package-root flag passed to the executable
    * used to run the script in this isolate.  This is the directory in which
    * Dart packages are looked up.
    *
    * If there is no --package-root flag, then the empty string is returned.
    */
   static String get packageRoot => _Platform.packageRoot;
 
   /**
    * Returns the version of the current Dart runtime.
+   *
+   * The returned `String` is formatted as the
+   * [semver](http://semver.org) version string of the current dart
+   * runtime, possibly followed by whitespace and other version and
+   * build details.
    */
   static String get version => _version;
 }