Document PackageStructure ownership when using PackageLoader

Since the object ownership for PackageStructure does not quite match what is generally applied (in an unwritten way, though perhaps we should consider documenting this as an intention), this patch adds documentation to explain that the ownership of PackageStructure instances returned by PackageLoader are retained by that class, and that they should never be deleted by others.

We may wish to eventually refactor this to return a reference to the structure instead of a pointer (to make this behaviour self-documenting)