docs: explain effect of smp_read_barrier_depends() on modern architectures

The documentation for smp_read_barrier_depends() does not mention the architectures
for which it is an optimization, for example ARM and PPC.  As a result, it is not
clear to the reader why one would use it.  Relegate Alpha to a footnote together
with other architectures where it is equivalent to smp_rmb().

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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