KVM: documentation: fix KVM_CAP_X2APIC_API information

The KVM_X2APIC_API_USE_32BIT_IDS feature applies to both
KVM_SET_GSI_ROUTING and KVM_SIGNAL_MSI, but was not mentioned in the
documentation for the latter ioctl.

Signed-off-by: Paolo Bonzini <[e-mail]>

Documentation/virtual/kvm/api.txt

···
 for the device that wrote the MSI message.  For PCI, this is usually a
 BFD identifier in the lower 16 bits.
 
-On x86, address_hi is ignored unless the KVM_CAP_X2APIC_API capability is
-enabled.  If it is enabled, address_hi bits 31-8 provide bits 31-8 of the
-destination id.  Bits 7-0 of address_hi must be zero.
+On x86, address_hi is ignored unless the KVM_X2APIC_API_USE_32BIT_IDS
+feature of KVM_CAP_X2APIC_API capability is enabled.  If it is enabled,
+address_hi bits 31-8 provide bits 31-8 of the destination id.  Bits 7-0 of
+address_hi must be zero.
 
 
 4.71 KVM_CREATE_PIT2