arm/arm64: KVM: vgic: Clarify and correct vgic documentation

The VGIC virtual distributor implementation documentation was written a
very long time ago, before the true nature of the beast had been
partially absorbed into my bloodstream.  Clarify the docs.

Plus, it fixes an actual bug.  ICFRn, pfff.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>