KVM: Add missing KVM_IRQFD API documentation

Signed-off-by: Alex Williamson <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Signed-off-by: Marcelo Tosatti <[e-mail]>

Documentation/virtual/kvm/api.txt

···
 PTE's RPN field (ie, it needs to be shifted left by 12 to OR it
 into the hash PTE second double word).
 
+4.75 KVM_IRQFD
+
+Capability: KVM_CAP_IRQFD
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_irqfd (in)
+Returns: 0 on success, -1 on error
+
+Allows setting an eventfd to directly trigger a guest interrupt.
+kvm_irqfd.fd specifies the file descriptor to use as the eventfd and
+kvm_irqfd.gsi specifies the irqchip pin toggled by this event.  When
+an event is tiggered on the eventfd, an interrupt is injected into
+the guest using the specified gsi pin.  The irqfd is removed using
+the KVM_IRQFD_FLAG_DEASSIGN flag, specifying both kvm_irqfd.fd
+and kvm_irqfd.gsi.
+
+
 5. The kvm_run structure
 ------------------------