Clarify what is eligible to be an owner

The section on ownership currently begins "Every object (value) in Rust has a single owner.".

I think it ought to tell the reader from the start what kind of thing is eligible to be an owner.

I think at that point I might be wondering "Are objects owned by threads? Memory allocators? Function activations? Other objects?".

The second paragraph tells us that "other objects" is one possibility, but that's a bit late and it's only part of the answer.