Probably in the same project as the main documentation (which incidentally doesn't exist yet). Just add a dedicated chapter in the user guide for now, we can always move it to a separate "Hibernate Search developpers" documentation later.

In particular:

• Explain the general layout (engine, mapper, backends, ...)
• Explain the bootstrap process of the POJO mapper
• Explain the specifics of each backend during bootstrap
• Explain how indexing works in the POJO mapper
• Explain how querying works in the POJO mapper
• Explain the specifics of each backend during runtime

And try to explain this wherever it fits best:

• The building process of PojoIndexingProcessors
• The building process of PojoImplicitReindexingResolvers
• The building and binding process of POJO bridges
• POJO model paths, in particular why there are separate type nodes below value nodes
• ... more?