Documentation: Add Helm Chart information about HostAliases and limitations

What does this MR do?

We've had a few support tickets come in and some repeated exploration among support engineers about the ability to provide custom /etc/hosts entries for the build pods created by the kubernetes executor. This docs MR adds a mention of the hostAliases: option that was added to the helm chart in this MR, and clarifies that it's currently limited to the runner pod itself, not the build/other job pods.

Related issues

Issue #2818 (closed) is in progress to allow for kubernetes HostAlias use in the job pods, similar to setting extra_hosts for the docker executor.

That issue is referenced in this docs MR as well.

Closes gitlab#216002 (closed)