scripts/documentation-file-ref-check: check tools/*/Documentation

Some files, like tools/memory-model/README has references to
a Documentation file that is locale to it. Handle references
that are relative to them too.

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