Guide: ownership

Whew. This section was so important, I saved it for last.

/cc @wycats, @nikomatsakis
Maybe you could expand on why even immutable pointers are subject to #4 above. If someone had asked me to guess, I would have said that those needn't be returned before the loaned pointer is returned because they're immutable. After mulling it over for a minute, I guess you don't want to deallocate the memory at the top of the borrow "tree" if there is someone further down still using it, so you have to collapse the tree leaf nodes first as you return. Or maybe I'm wrong.