The KEP (Kubernetes Enhancement Proposal) is linked to in the PR [1]. From the summary:
> Sidecar containers are a new type of containers that start among the Init containers, run through the lifecycle of the Pod and don’t block pod termination. Kubelet makes a best effort to keep them alive and running while other containers are running.
> Sidecar containers are a new type of containers that start among the Init containers, run through the lifecycle of the Pod and don’t block pod termination. Kubelet makes a best effort to keep them alive and running while other containers are running.
[1] https://github.com/kubernetes/enhancements/tree/master/keps/...