docs: Add an SPDX header to kernel-doc

Add the SPDX header while I'm in the neighborhood.  The source itself just
says "GNU General Public License", but it also refers people to the COPYING
file for further information.  Since COPYING says 2.0-only, that is what I
have put into the header.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>