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'm not a language prescriptivist, but I give zero weight to any opinion ESR holds, sorry.

Anyway, currently, the standard is comma inside. This isn't the place to argue about it.