device_delete_child.9: Reference device_detach.9 and BUS_CHILD_DELETED.9

Refer to device_detach(9) for the description of detaching a device.
Note that descendants of the child device are also deleted (though in
theory a proper detach routine should have already deleted these).
Finally, note that BUS_CHILD_DELETED(9) is invoked on each deleted
device.

Differential Revision: https://reviews.freebsd.org/D48365