vga_switcheroo: Document _ALL_ the things!

This adds an "Overview" DOC section plus two DOC sections for the modes
of use ("Manual switching and manual power control" and "Driver power
control").

Also included is kernel-doc for all public functions, structs and enums.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>