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>

Documentation/core-api/kernel-api.rst

@@ -412,3 +412,15 @@ Read-Copy Update (RCU)
 .. kernel-doc:: include/linux/rcu_sync.h
 
 .. kernel-doc:: kernel/rcu/sync.c
+
+.. kernel-doc:: kernel/rcu/tasks.h
+
+.. kernel-doc:: kernel/rcu/tree_stall.h
+
+.. kernel-doc:: include/linux/rcupdate_trace.h
+
+.. kernel-doc:: include/linux/rcupdate_wait.h
+
+.. kernel-doc:: include/linux/rcuref.h
+
+.. kernel-doc:: include/linux/rcutree.h