scripts: kernel-doc: try to use c:function if possible

There are a few namespace clashes by using c:macro everywhere:

basically, when using it, we can't have something like:

	.. c:struct:: pwm_capture

	.. c:macro:: pwm_capture

So, we need to use, instead:

	.. c:function:: int pwm_capture (struct pwm_device * pwm, struct pwm_capture * result, unsigned long timeout)

for the function declaration.

The kernel-doc change was proposed by Jakob Lykke Andersen here:

	https://github.com/jakobandersen/linux_docs/commit/6fd2076ec001cca7466857493cd678df4dfe4a65



Although I did a different implementation.

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