In AS7 under Projects click on Re-Configure Atmel Start Project, the Start configurator window opens. Clicking on User guide will take you to the ASF4 API Reference Manual.
You will find User guide in every component as well but it only gives a summary of the device itself. Enjoy!
Link to a pdf of the ASF4 API Reference Manual.
EDIT: At the START page, http://start.atmel.com/, there are the What's New and Help buttons at the top of the page that provide access to the START User guide, ASF4 API Reference Manual, Microchip Support, and the Changelog.