docs: kernel_include.py: fix an issue when O= is used

docs: kernel_include.py: fix an issue when O= is used

As reported by Stephen, building docs with O= is now
broken. Fix it by ensuring that it will seek files under
Kernel source tree.

The original logic was defined to accept including files
under Documentation/output. The new logic doesn't need it
anymore for media, but it might still be useful to preserve
the previous behavior. So, I ended preserving it.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20250901142639.4de35a11@canb.auug.org.au/
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/da91980ce42f31730dc982920167b2757b9d2769.1756732363.git.mchehab+huawei@kernel.org