Given that indexes are critical to the good performance of queries in ModeShape 4, I've looked for documentation on this topic.
But I couldn't find any reference documentation that fully explains how to configure indexes.
The next best thing that I've tracked down is the MODE-1260 issue.
I think that would be a good starting point for such reference documentation.

There's still a couple of things that needs to covered, e.g.:

• what's the function of the "nodeType" attribute?
  • what if it's not specified?
• what's the syntax of the "columns" attribute?
  • what if the type mentioned there is different from the actual property type?
  • is there some peculiarities to be aware of for some JCR types? (e.g. should BOOLEAN be treated as "enumerated"?)
• what's the use of a "nodetype" index?
  • it's mentioned as "experimental" and I couldn't find any example of use