scripts: documentation-file-ref-check: improve :doc: handling

There are some issues at the script with regards to :doc:
tags:

- It doesn't escape files under Documentation/sphinx,
  leading to false positives;
- It doesn't handle root URLs, like :doc:`/x86/boot`;
- It doesn't output the file with a bad reference.

Address those things, in order to remove false positives
from the list of problems.

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