drm: Documentation style guide

Every time I type or review docs this seems a bit different. Try to
document the common style so we can try to unify at least new docs.

v2: Spelling fixes from Pierre, Laurent and Jani.

v3: More spelling fixes from Lukas.

Cc: Pierre Moreau <pierre.morrow@free.fr>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Lukas Wunner <lukas@wunner.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449564561-3896-1-git-send-email-daniel.vetter@ffwll.ch