Document actual within_frame support

Currently it's specified that `within_frame` supports frame id as a parameter. However, currently:
* RackTest doesn't support `within_frame`
* Webdriver [supports](http://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/frame) frame id, name, index and WebElement
* Capybara-Webkit and Poltergeist support only name and index

So currently id is supported by one driver. Name and index are supported by 3 drivers. I changed documenation to clarify it.