doc: Fix list: rcu_access_pointer() is not lockdep-checked

doc: Fix list: rcu_access_pointer() is not lockdep-checked

The rcu_access_pointer() macro does not consult lockdep by design because
it is intended to be used outside of RCU read-side critical sections.
This commit therefore makes a separate list for it in whatisRCU.rst.

Similarly, RCU_LOCKDEP_WARN(), rcu_sleep_check(), and RCU_NONIDLE()
do not do anything with pointer access.  This commit therefore creates
a separate utility-API list for them.

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