scripts: kernel-doc: make it more compatible with Sphinx 3.x

With Sphinx 3.x, the ".. c:type:" tag was changed to accept either:

	.. c:type:: typedef-like declaration
	.. c:type:: name

Using it for other types (including functions) don't work anymore.

So, there are newer tags for macro, enum, struct, union, and others,
which doesn't exist on older versions.

Add a check for the Sphinx version and change the produced tags
accordingly.

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