Add doc for exporting config from a groovy script

[its-dave]

Your checklist for this pull request

🚨 Please review the guidelines for contributing to this repository.

• [x] Make sure you are requesting to pull a topic/feature/bugfix branch (right side) and not your master branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or in Jenkins JIRA
• [ ] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Did you provide a test-case? That demonstrates feature works or fixes the issue.

[timja]

Hmm any thoughts @jetersen ?

I guess we could document this but its an advanced only usecase and it could break, NoExternalUse means its a private API and can be changed at any time.

[its-dave]

That seems reasonable, it makes sense to treat it the same way as the groovy script method of reloading the config