docs: cdomain.py: extend it to handle new Sphinx 3.x tags

While most of the C domain parsing is done via kernel-doc,
some RST files use C domain tags directly.

While several of them can be removed for Sphinx < 3.0, due
to automarkup.py, and several others that could be
converted into kernel-doc markups, changes like that are
time-consuming, and may not fit all cases.

As we already have the cdomain.py for handing backward
compatibility with Sphinx versions below 3.0, let's
make it more complete, in order to cover any usage of the
newer tags outside kernel-doc.

This way, it should be feasible to use the new tags inside
the Kernel tree, without losing backward compatibility.

This should allow fixing the remaining warnings with
the Kernel tags.

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