docs: Document limitation for kernels without netns cookie
docs: Document limitation for kernels without netns cookie

On kernels without support for netns cookies, services are accessible
via the loopback address from all pods. That is because Cilium cannot
distinguish between host and pod namespaces in that case.

Suggested-by: Daniel Borkmann <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@iogearbox.net>
Signed-off-by: Paul Chaignon <a027184a55211cd23e3f3094f1fdc728df5e0500@cilium.io>

tklauser/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium