docs/memory-barriers.txt: Remove remaining references to mmiowb()

     - Compiler barrier.

     - CPU memory barriers.

     - MMIO write barrier.

 (*) Implicit kernel memory barriers.

 (*) Inter-CPU acquiring barrier effects.

     - Acquires vs memory accesses.

     - Acquires vs I/O accesses.

 (*) Where are memory barriers needed?

     happen before it completes.

     The use of ACQUIRE and RELEASE operations generally precludes the need

     for other sorts of memory barrier (but note the exceptions mentioned in

     the subsection "MMIO write barrier").  In addition, a RELEASE+ACQUIRE

     pair is -not- guaranteed to act as a full memory barrier.  However, after

     an ACQUIRE on a given variable, all memory accesses preceding any prior

     for other sorts of memory barrier.  In addition, a RELEASE+ACQUIRE pair is

     -not- guaranteed to act as a full memory barrier.  However, after an

     ACQUIRE on a given variable, all memory accesses preceding any prior

     RELEASE on that same variable are guaranteed to be visible.  In other

     words, within a given variable's critical section, all accesses of all

     previous critical sections for that variable are guaranteed to have

  (*) CPU memory barriers.

  (*) MMIO write barrier.

COMPILER BARRIER

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