rculist: Unify documentation about missing list_empty_rcu()

We have two separate sections that talk about why list_empty_rcu()
is not needed, so this commit consolidates them.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
[ paulmck: The usual wordsmithing. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>