docs: kvm: riscv: document EBUSY in KVM_SET_ONE_REG

The EBUSY errno is being used for KVM_SET_ONE_REG as a way to tell
userspace that a given reg can't be changed after the vcpu started.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>

Author: danielhb

Documentation/virt/kvm/api.rst

@@ -2259,6 +2259,8 @@ Errors:
   EINVAL   invalid register ID, or no such register or used with VMs in
            protected virtualization mode on s390
   EPERM    (arm64) register access not allowed before vcpu finalization
+  EBUSY    (riscv) changing register value not allowed after the vcpu
+           has run at least once
   ======   ============================================================
 
 (These error codes are indicative only: do not rely on a specific error