KVM: arm64: Document vCPU event ioctls as requiring init'ed vCPU

KVM rejects calls to KVM_{GET,SET}_VCPU_EVENTS for an uninitialized vCPU
as of commit cc96679f3c03 ("KVM: arm64: Prevent access to vCPU events
before init"). Update the corresponding API documentation.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: oupton

Documentation/virt/kvm/api.rst

@@ -1229,6 +1229,9 @@ It is not possible to read back a pending external abort (injected via
 KVM_SET_VCPU_EVENTS or otherwise) because such an exception is always delivered
 directly to the virtual CPU).
 
+Calling this ioctl on a vCPU that hasn't been initialized will return
+-ENOEXEC.
+
 ::
 
   struct kvm_vcpu_events {
@@ -1309,6 +1312,8 @@ exceptions by manipulating individual registers using the KVM_SET_ONE_REG API.
 
 See KVM_GET_VCPU_EVENTS for the data structure.
 
+Calling this ioctl on a vCPU that hasn't been initialized will return
+-ENOEXEC.
 
 4.33 KVM_GET_DEBUGREGS
 ----------------------