scripts/documentation-file-ref-check: improve tools ref handling

There's a false positive on perf/util:

	tools/perf/util/s390-cpumsf.c: Documentation/perf.data-file-format.txt

The file is there at tools/perf/Documentation/, but the logic
with detects relative documentation references inside tools is
not capable of detecting it.

So, improve it.

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