Guide: ownership

Whew. This section was so important, I saved it for last.

/cc @wycats, @nikomatsakis
FWIW, allocation isn't the only thing that needs destruction/clean-up, e.g. for [`RefCell`](http://doc.rust-lang.org/master/std/cell/struct.RefCell.html), `borrow_mut()` constructs a [`RefMut`](http://doc.rust-lang.org/master/std/cell/struct.RefMut.html) by (in a sense) taking ownership of the 'borrow' resource, with the destructor freeing it.