Make multi-field search a first-class citizen

### Description

Searching across multiple fields is a very frequent need, e.g. both Solr and Elasticsearch allow searching a query string on multiple fields, yet Lucene doesn't make it easy.

- [x] #14236
- [ ] #14262
- [ ] Improve docs wrt robust approaches for combining scores across multiple fields

Start by reading the linked issues #14236 and #14262 to understand which parts of multi-field search are already addressed and what remains. The open work described here is to improve the documentation of robust approaches for combining scores across multiple fields; done means those approaches are clearly documented.

Written by the indexing model from the issue text.