Document multi-site/multi-index search capabilities

[rmccue]

There's essentially no documentation either in our docs or in ElasticPress' docs about how to search across multiple sites/indexes. This is a pretty common use case we've seen, so we should ensure that we have solid, robust docs on how to do so.

• As a developer, I want to understand how to search across every site so that I can build a global search for my network
• As a developer, I want to understand how to search across multiple sites (but not all) so that I can build search functionality for multiple related sites