Automatic merge from submit-queue

CRI: Clarify User in CRI.

Addressed https://github.com/kubernetes/kubernetes/pull/36423#issuecomment-259343135.

This PR clarifies the user related fields in CRI.

One question is that:
What is the meaning of the `run_as_user` field in `LinuxSandboxSecurityContext`?
* **Is it user on the host?** Then it doesn't make sense, user shouldn't care about what users are on the host.
* **Is it user inside the infra container image?** This is how the field is currently used. However, Infra container is docker specific, I'm not sure whether we should expose this in CRI.
* **Is it the default user inside the pod?** It tells runtime that if there is a container (infra container, or some other helper containers like streaming container etc.), if their `user` is not specified, use the default "sandbox user". Then how can we guarantee that infra or helper container image have the `user`?
* **It doesn't make sense?** If we remove it, we are relying on the shim to set right user (maybe always root) for infra or helper containers (if there will be any in the future), I'm not sure whether this is what we expect.

@yujuhong @feiskyer @jonboulle @yifan-gu 
/cc @kubernetes/sig-node