Update k8s strategy documentation

[cybrox]

Summary of changes

Added a short example of rel/env.sh.eex to make the kubernetes strategy in :mode :dns work with releases that have been assembled with mix release.

Also changed the doc block to not handle escape characters as the \ in the sed command got lost.

Checklist

• [x] New functions have typespecs, changed functions were updated
• [x] Same for documentation, including moduledocs
• [x] Tests were added or updated to cover changes
• [x] Commits were squashed into a single coherent commit
• [x] Notes added to CHANGELOG file which describe changes at a high-level