documentation: Add lockless_dereference()

The recently added lockless_dereference() macro is not present in the
Documentation/ directory, so this commit fixes that.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>