Allow query to return documents with attachments

Currently the `find` query system doesn't give the option to return documents with their attachments.

Would it be good to add an `attachments:true` option.

Start by tracing the `find` query system and how it currently handles documents and attachments. Define the expected behavior of an `attachments:true` option, then identify the relevant tests and update them so queries with and without the option have clear, verifiable results.

Written by the indexing model from the issue text.