scripts/documentation-file-ref-check: teach about .txt -> .yaml renames

At DT, files are being renamed to jason. Teach the script how to
handle such renames when used in fix mode.

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