docs: remove :c:func: from refcount-vs-atomic.rst

As of 5.3, the automarkup extension will do the right thing with function()
notation, so we don't need to clutter the text with :c:func: invocations.
So remove them.

Looking at the generated output reveals that we lack kerneldoc coverage for
much of this API, but that's a separate problem.

Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>