Ownership guide

This is a work in progress, but this should get *extensive* review, so I'm putting it up early and often.

This is the start of a draft of the new 'ownership guide,' which explains ownership, borrowing, etc. I'm feeling better about this framing than last time's, but we'll see.
I thought bjz was referring to the fact that you're adding one to the value in the box, throwing away the result, then returning the unmodified box.