Document "internal" options

## Is your feature request related to a problem?
Some options are tagged as "internal" and not documented, and after discussion we've decided it's better to ...

## Describe the solution you'd like
... document them, but carefully explain why they shouldn’t be used, and keep them lower down on the page so the more used options are seen first. Maybe move the "internal" tag to "advanced" or similar

## Describe alternatives you've considered
Leaving it as is

## Additional context
Discussion was generated from [this comment](https://github.com/elastic/apm-agent-java/pull/2148)