Add reference for `KUBERNETES_EXEC_INFO`

**This is a Feature Request**

**What would you like to be added**
Kubernetes can set an environment variable, `KUBERNETES_EXEC_INFO`. See https://k8s.io/docs/reference/access-authn-authz/authentication/#client-go-credential-plugins

I'd like us to have documentation for what the contents of that variable are and what other parties should expect.

**Why is this needed**
- This would help people write authentication plugins
- This would help people implement Kubernetes client libraries
- This would help people implement single-purpose Kubernetes client software

**Comments**
I spotted the lack of docs when trying to troubleshoot a subcommand of the `aws` CLI tool.

/language en
I'm not sure which SIG would own this.

Start with the existing Kubernetes authentication reference linked in the issue and determine where the KUBERNETES_EXEC_INFO reference belongs. Document the variable's contents and the expectations for authentication plugins and client implementations; the work is done when those details are available in the reference documentation.

Written by the indexing model from the issue text.