Document offloading limiations

Discussed in https://github.com/kubernetes/minikube/pull/22962
- All VMs on the same vmnet bridge must enable offloading. Mixing VMs with and without offloading causes the bridge to disable TSO, dropping TX to ~1.6 Gbit/s.
- Pod-to-VM RX is severely degraded (10.3 Gbit/s → 2.3 Mbit/s), making it unusable for that traffic pattern.