rculist.h: docs: Fix wrong function summary

The brief summary in the docstring for function list_next_or_null_rcu()
states that the function is supposed to provide the "first" member of a
list, whereas in truth it returns the next member.

Change the docstring so it describes what the function actually does.

Signed-off-by: Philipp Stanner <pstanner@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>