documentation: Add acquire/release barriers to pairing rules

When dealing with CPU-CPU interactions, certain types of memory barrier should

always be paired.  A lack of appropriate pairing is almost certainly an error.

A write barrier should always be paired with a data dependency barrier or read

barrier, though a general barrier would also be viable.  Similarly a read

barrier or a data dependency barrier should always be paired with at least an

write barrier, though, again, a general barrier is viable:

General barriers pair with each other, though they also pair with

most other types of barriers, albeit without transitivity.  An acquire

barrier pairs with a release barrier, but both may also pair with other

barriers, including of course general barriers.  A write barrier pairs

with a data dependency barrier, an acquire barrier, a release barrier,

a read barrier, or a general barrier.  Similarly a read barrier or a

data dependency barrier pairs with a write barrier, an acquire barrier,

a release barrier, or a general barrier:

	CPU 1		      CPU 2

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