Docs/ABI/testing: Add VDUSE sysfs interface ABI document

This adds missing documentation for VDUSE sysfs interface ABI
under Documentation/ABI/testing.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20220524115143.187-1-xieyongji@bytedance.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>