Documentation: userspace-api: iommufd: Update HW QUEUE

With the introduction of the new object and its infrastructure, update the
doc to reflect that.

Link: https://patch.msgid.link/r/caa3ddc0d9bacf05c5b3e02c5f306ff3172cc54d.1752126748.git.nicolinc@nvidia.com


Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>