docbook: add futexes to kernel-locking docbook

Add Fast User Mutexes (futexes) to kernel-locking docbook.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/DocBook/kernel-locking.tmpl

@@ -1955,12 +1955,17 @@ machines due to caching.
    </sect1>
   </chapter>
 
-  <chapter id="apiref">
+  <chapter id="apiref-mutex">
    <title>Mutex API reference</title>
 !Iinclude/linux/mutex.h
 !Ekernel/mutex.c
   </chapter>
 
+  <chapter id="apiref-futex">
+   <title>Futex API reference</title>
+!Ikernel/futex.c
+  </chapter>
+
   <chapter id="references">
    <title>Further reading</title>