Documentation: KVM: Update GICv3 docs for GICv5 hosts

GICv5 hosts optionally include FEAT_GCIE_LEGACY, which allows them to
execute GICv3-based VMs on GICv5 hardware. Update the GICv3
documentation to reflect this now that GICv3 guests are supports on
compatible GICv5 hosts.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Sticklyman1936

Documentation/virt/kvm/devices/arm-vgic-v3.rst

@@ -13,7 +13,8 @@ will act as the VM interrupt controller, requiring emulated user-space devices
 to inject interrupts to the VGIC instead of directly to CPUs.  It is not
 possible to create both a GICv3 and GICv2 on the same VM.
 
-Creating a guest GICv3 device requires a host GICv3 as well.
+Creating a guest GICv3 device requires a host GICv3 host, or a GICv5 host with
+support for FEAT_GCIE_LEGACY.
 
 
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