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

Documentation/gpu/drm-kms.rst

@@ -293,6 +293,12 @@ Tile Group Property
 .. kernel-doc:: drivers/gpu/drm/drm_connector.c
    :doc: Tile group
 
+Explicit Fencing Properties
+---------------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
+   :doc: explicit fencing properties
+
 Existing KMS Properties
 -----------------------