The multipart API is not documented

Based on answers to questions like "is it possible to upload documents and attachments at the same time?" in IRC, it seems that there is a multipart API that allows that sort of thing, but I've been told that it's not documented (and haven't been able to find any mention of it via searching).

• How can it be invoked?
• Can it be used instead of bulk_docs? Ie. for multiple documents at once.
• What are the _rev semantics (ATM each attachment increments the _rev by one; would it be possible to have a document at _rev 1 with all attachments?)?