Improve KeyEvent documentation.

tools/dom/src/dart2js_KeyEvent.dart

Index: tools/dom/src/dart2js_KeyEvent.dart
diff --git a/tools/dom/src/dart2js_KeyEvent.dart b/tools/dom/src/dart2js_KeyEvent.dart
index f3be576c6e5379c72a87ae44f5ff9b28a4117209..e8124e043b1a132230be77ce6db3287649604d40 100644
--- a/tools/dom/src/dart2js_KeyEvent.dart
+++ b/tools/dom/src/dart2js_KeyEvent.dart
@@ -6,6 +6,21 @@
  * KeyEvent tries to provide a higher level, more polished keyboard event
  * information on top of the "raw" [KeyboardEvent].
  *
+ * The mechanics of using KeyEvents is a little different from the underlying
+ * [KeyboardEvent]. To use KeyEvents, you need to create a stream and then add
+ * KeyEvents to the stream, rather than using the [EventTarget.dispatchEvent].
+ * Here's an example usage:
+ *
+ *     // Initialize a stream for the KeyEvents:
+ *     var stream = KeyEvent.keyPressEvent.forTarget(document.body);
+ *     // Start listening to the stream of KeyEvents.
+ *     stream.listen((keyEvent) =>
+ *         window.console.log('KeyPress event detected ${keyEvent.charCode}'));
+ *     ...
+ *     // Add a new KeyEvent of someone pressing the 'A' key to the stream so
+ *     // listeners can know a KeyEvent happened.
+ *     stream.add(new KeyEvent('keypress', keyCode: 65, charCode: 97));
+ *
  * This class is very much a work in progress, and we'd love to get information
  * on how we can make this class work with as many international keyboards as
  * possible. Bugs welcome!