documentation/kvm: additional explanations on KVM_SET_BOOT_CPU_ID

The ioctl KVM_SET_BOOT_CPU_ID fails when called after vcpu creation.
Add this explanation in the documentation.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20210319091650.11967-1-eesposit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>