update elements and fix some bugs

lib/iron_validatable_behavior.dart

Index: lib/iron_validatable_behavior.dart
diff --git a/lib/iron_validatable_behavior.dart b/lib/iron_validatable_behavior.dart
index 1ec3e95584126675b16900b3d4a7996b400bbc54..da4161acd25cbf30099b55b2cbb0e9483c408de8 100644
--- a/lib/iron_validatable_behavior.dart
+++ b/lib/iron_validatable_behavior.dart
@@ -10,7 +10,20 @@ import 'package:web_components/web_components.dart';
 import 'package:polymer_interop/polymer_interop.dart';
 import 'iron_meta.dart';
 
-/// Use `Polymer.IronValidatableBehavior` to implement an element that validates user input.
+/// `Use Polymer.IronValidatableBehavior` to implement an element that validates user input.
+/// Use the related `Polymer.IronValidatorBehavior` to add custom validation logic to an iron-input.
+///
+/// By default, an `<iron-form>` element validates its fields when the user presses the submit button.
+/// To validate a form imperatively, call the form's `validate()` method, which in turn will
+/// call `validate()` on all its children. By using `Polymer.IronValidatableBehavior`, your
+/// custom element will get a public `validate()`, which
+/// will return the validity of the element, and a corresponding `invalid` attribute,
+/// which can be used for styling.
+///
+/// To implement the custom validation logic of your element, you must override
+/// the protected `_getValidity()` method of this behaviour, rather than `validate()`.
+/// See [this](https://github.com/PolymerElements/iron-form/blob/master/demo/simple-element.html)
+/// for an example.
 ///
 /// ### Accessibility
 ///