doc: Update rcu_access_pointer() advice in rcu_dereference.rst

This commit updates the rcu_access_pointer() advice, noting that its
return value should not be assigned to a local variable, and also noting
that there is little point in using rcu_access_pointer() within an RCU
read-side critical section.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>