Document index building and options more thoroughly

[alecgrieser]

The OnlineIndexer already has javadocs, but given how important indexing is, it would probably make sense to have separate documentation walking someone through the process as well as explaining some of the ways that it can be adjusted for performance's sake. Advanced topics could include things like parallelizing the index build (through chunking up the records into multiple ranges, etc.).

[nblintao]

cc myself

[nblintao]

@alecgrieser had already put something here https://forums.foundationdb.org/t/online-indexing-on-large-databases/2391/2