docs: conf.py: change the Sphinx 3.x+ text

After the build fix patches, it is believed that the output
with Sphinx 3.x won't be a problem. Still, the C domain
support was re-written, and this can have caused hidden
issues.

So, let's keep the warning, changing it to a lighter
warning text.

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