KVM: Document weakly ordered architecture requirements for dirty ring

Now that the kernel can expose to userspace that its dirty ring
management relies on explicit ordering, document these new requirements
for VMMs to do the right thing.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20220926145120.27974-5-maz@kernel.org