apple-gmux: Sphinxify docs

Convert asciidoc-formatted docs to rst in accordance with Jonathan's and
Jani's effort to use sphinx for kernel-doc rendering in 4.8.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/4c1b29986fa77772156b1af0c965d3799e43a47b.1467628307.git.lukas@wunner.de