KVM: arm/arm64: Add GICV3 pending table save API documentation

Add description for how to save GICV3 LPI pending bit into
guest RAM pending tables.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Acked-by: Christoffer Dall <cdall@linaro.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>