vga_switcheroo: Prettify documentation

Fix indentation of vga_switcheroo sections in gpu.tmpl.

Change section type of API documentation from "chapter" to "sect1"
so that the individual functions no longer clutter up the ToC.
Group together under a new "API" chapter.

Fix wording "heretoforth" -> "henceforth".

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