Documentation/filesystem/dax: DAX on virtiofs

Record DAX on virtiofs and the semantic difference with that on ext4
and xfs.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>