Improve Documentation

[indolering]

The docs are light on details about how this should be used. It would be nice if there were some examples for usage in both parent and child elements. I've added questions I had while reading through the API documentation:

assignParentResizable

• Is this mandatory for any element that has a parent that implements iron-resizable?
• Should it be done in attached or another lifecycle callback?

notifyResize

• While the docs state that this can be called manually, the example calls it during a lifecycle callback (this.async(this.notifyResize, 1). Is this mandatory or a good idea or ...?

[mkazlauskas]

It feels like it should notifyResize during lifecycle callback inside the behavior itself since it seems to be breaking 1.0 functionality.