drm/syncobj: some kerneldoc polish

Complete a few missing bits, fix up the existing xcross-references and
add a bunch more.

v2: Fix typos (Alex).

Cc: Dave Airlie <airlied@gmail.com> via lists.freedesktop.org
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171214203054.20141-4-daniel.vetter@ffwll.ch