doc: Drop doubled words from RCU requirements documentation

   this sort of thing.

#. If a CPU is in a portion of the kernel that is absolutely positively

   no-joking guaranteed to never execute any RCU read-side critical

   sections, and RCU believes this CPU to to be idle, no problem. This

   sections, and RCU believes this CPU to be idle, no problem. This

   sort of thing is used by some architectures for light-weight

   exception handlers, which can then avoid the overhead of

   ``rcu_irq_enter()`` and ``rcu_irq_exit()`` at exception entry and

not have this property, given that any point in the code outside of an

RCU read-side critical section can be a quiescent state. Therefore,

*RCU-sched* was created, which follows “classic” RCU in that an

RCU-sched grace period waits for for pre-existing interrupt and NMI

RCU-sched grace period waits for pre-existing interrupt and NMI

handlers. In kernels built with ``CONFIG_PREEMPT=n``, the RCU and

RCU-sched APIs have identical implementations, while kernels built with

``CONFIG_PREEMPT=y`` provide a separate implementation for each.