doc: Update the comparisons rule in rcu_dereference.txt

When an RCU-protected pointer is fetched but never dereferenced
rcu_access_pointer() should be used in place of rcu_dereference().
This commit explicitly records this very fact in Documentation/
RCU/rcu_dereference.txt, in order to prevent the usage of
rcu_dereference() in comparisons.

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