Search 6 groundwork - Document the configuration options in Search 6

The idea would be to document everything that is currently available, so that we can easily document any option we add later.

To be documented both:

• In the javadoc (summary of what it's for, accepted types)

• In the reference documentation (same as above + more details about use cases and links to relevant parts of the reference documentation)

In particular the lifecycle strategy option for Elasticsearch is currently undocumented.