thunderbolt: Update documentation with the USB4 information

Update user's and administrator's guide to mention USB4, how it relates
to Thunderbolt and and how it is supported in Linux.

While there add the missing SPDX identifier to the document.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Link: https://lore.kernel.org/r/20191217123345.31850-10-mika.westerberg@linux.intel.com


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