Guide: ownership

Whew. This section was so important, I saved it for last.

/cc @wycats, @nikomatsakis
1, 2, and 3 are privileges. Only 4 is a rule. I don't think they should all be in a list together.

Furthermore, 4 written as a rule that applies to 3, but I think it's just a special case of borrows having to have a shorter lifetime than the thing they are borrowing from (whether the lender is an owner or an intermediate borrower). It would be simpler to explain that you, the borrower, are required to give it back.