drm/fences: add DOC: for explicit fencing

Document IN_FENCE_FD and OUT_FENCE_PTR properties.

v2: incorporate comments from Daniel Vetter

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
[danvet: s/async/nonblocking/ atomic commits.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1479773488-15048-1-git-send-email-gustavo@padovan.org