KVM: x86: document KVM_REQ_PENDING_TIMER usage

Document that KVM_REQ_PENDING_TIMER is implicitly used during guest
entry.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/timer.c

@@ -12,7 +12,8 @@ static int __kvm_timer_fn(struct kvm_vcpu *vcpu, struct kvm_timer *ktimer)
 	/*
 	 * There is a race window between reading and incrementing, but we do
 	 * not care about potentially loosing timer events in the !reinject
-	 * case anyway.
+	 * case anyway. Note: KVM_REQ_PENDING_TIMER is implicitly checked
+	 * in vcpu_enter_guest.
 	 */
 	if (ktimer->reinject || !atomic_read(&ktimer->pending)) {
 		atomic_inc(&ktimer->pending);