Adds documentation for the Joomla\Form\Form class

Sorry folks I've probably completely stuffed this PR.  It's my first attempt at this but just wanted to contribute back some documentation.  I notice when I compare I've got some of dongilbert's changes in here as well.

This adds some documentation about how to use for Joomla\Form\Form class, create XML and form validation rules.

Cheers,
Eric.