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 <pbonzini@redhat.com>