KVM: arm64: Document the behaviour of S1PTW faults on RO memslots

KVM: arm64: Document the behaviour of S1PTW faults on RO memslots

Although the KVM API says that a write to a RO memslot must result
in a KVM_EXIT_MMIO describing the write, the arm64 architecture
doesn't provide the *data* written by a Stage-1 page table walk
(we only get the address).

Since there isn't much userspace can do with so little information
anyway, document the fact that such an access results in a guest
exception, not an exit. This is consistent with the guest being
terminally broken anyway.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>