Doc/clarify private registry access

Merged Thiago Figueiró requested to merge doc/clarify-private-registry-access into master

What does this MR do and why?

A forum user had trouble understanding why a private container image was still available to a CI build that should not have had access to it.

They found their own answer in our documentation, and responded in their post with a suggestion to improve our docs: https://forum.gitlab.com/t/is-any-user-able-to-pull-any-container-registry-image/77887/1

This MR provides two links to https://docs.gitlab.com/runner/executors/docker.html#when-not-to-use-the-if-not-present-pull-policy from locations that are likely to be seen when users are learning about private image permissions.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Edited by Thiago Figueiró