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>

Documentation/virtual/kvm/devices/arm-vgic-v3.txt

@@ -167,11 +167,17 @@ Groups:
     KVM_DEV_ARM_VGIC_CTRL_INIT
       request the initialization of the VGIC, no additional parameter in
       kvm_device_attr.addr.
+    KVM_DEV_ARM_VGIC_SAVE_PENDING_TABLES
+      save all LPI pending bits into guest RAM pending tables.
+
+      The first kB of the pending table is not altered by this operation.
   Errors:
     -ENXIO: VGIC not properly configured as required prior to calling
      this attribute
     -ENODEV: no online VCPU
     -ENOMEM: memory shortage when allocating vgic internal data
+    -EFAULT: Invalid guest ram access
+    -EBUSY:  One or more VCPUS are running
 
 
   KVM_DEV_ARM_VGIC_GRP_LEVEL_INFO