Solr 8.0 will have nicer support for nested child documents than its predecessors. This should be documented in one place in the ref guide (to the extent that makes sense). Users need to the schema ramifications (incl. special fields and that some aspects are optional and when), what a nested document "looks like" (XML, JSON, SolrJ), how to use the child doc transformer, how to use block join queries, and get some overview of how this all works. Maybe mention some plausible future enhancements / direction this is going in (e.g. path query language?). Some of this is already done but it's in various places and could be moved. Unlike other features which conveniently fit into one spot in the documentation (like a query parser), this is a more complex issue that has multiple aspects – more "cross-cutting", and so IMO doesn't belong in the current doc pigeon holes.