scripts: kernel-doc: use a less pedantic markup for funcs on Sphinx 3.x

Unfortunately, Sphinx 3.x parser for c functions is too pedantic:

	https://github.com/sphinx-doc/sphinx/issues/8241



While it could be relaxed with some configurations, there are
several corner cases that it would make it hard to maintain,
and will require teaching conf.py about several macros.

So, let's instead use the :c:macro notation. This will
produce an output that it is not as nice as currently, but it
should still be acceptable, and will provide cross-references,
removing thousands of warnings when building with newer
versions of Sphinx.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>