Add the RCU docs to the core-api manual

We should really move the RCU directory there as well, but that can wait
for another day.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/index.rst

@@ -35,6 +35,7 @@ Core utilities
    boot-time-mm
    memory-hotplug
    protection-keys
+   ../RCU/index
 
 
 Interfaces for kernel debugging