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 <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Luis Chamberlain <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Liam Beguin <[e-mail]>
Cc: <[e-mail]>

Documentation/core-api/kernel-api.rst

···
 .. 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