rcu: Add more RCU files to kernel-api.rst

Recent changes and additions to RCU have not been reflected in
Documentation/core-api/kernel-api.rst, which makes it harder to find
the kernel-doc headers in recently added RCU files.

Therefore, add those files.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Liam Beguin <liambeguin@gmail.com>
Cc: <linux-doc@vger.kernel.org>