doc: No longer allowed to use rcu_dereference on non-pointers

There are too many ways for the compiler to optimize (that is, break)
dependencies carried via integer values, so it is now permissible to
carry dependencies only via pointers.  This commit catches up some of
the documentation on this point.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>