drm/doc: document fallback behaviour for atomic events

Worst case if the hw can't support completion signalling in a
race-free way we want the event to be too late, not too early.

Text adapted from a proposal from Laurent - the other side of how to
make hw work correctly where it's possible is imo already sufficiently
documented.

v2: Review from Laurent.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308141257.12119-7-daniel.vetter@ffwll.ch