scripts/documentation-file-ref-check: don't parse Next/ dir

If one tries to run this script under linux-next, it would
hit lots of false-positives, due to the tree merges that
are stored under the Next/ directory.

So, add a logic to ignore it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>