Documentation: kvm/sev: clarify usage of KVM_MEMORY_ENCRYPT_OP

Explain that it operates on the VM file descriptor, and also clarify how
detection of SEV operates on old kernels predating commit 2da1ed62
("KVM: SVM: document KVM_MEM_ENCRYPT_OP, let userspace detect if SEV
is available").

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>