Add doc for exporting config from a groovy script

[its-dave]

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[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