docs: conf.py: rename some vars at latex_documents logic

docs: conf.py: rename some vars at latex_documents logic

Currently, the logic uses fn and doc vars, but they don't
properly describe what such vars do.

Make them clearer:
	- fname: points to the file name to search (index.rst);
	- doc: contains the name of the LaTeX or PDF doc to
	  be produced.

With that, the checks for SPHINXDIRS and for subdirs will
be more coherent.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/b030d9bc53550905adbe9367b2a3915d7331b4c5.1755763127.git.mchehab+huawei@kernel.org