Guide: ownership

Whew. This section was so important, I saved it for last.

/cc @wycats, @nikomatsakis
I think it'd be better to write foo() as `fn foo(x: &int) { }`. Then it's clear that "and then gives it right back" is referring to something more subtle than the function returning a value. And, saying that a completely empty function body does this will have a much greater effect.