rust-pyo3-sharedref: demonstrate SharedByPyObject borrow unsafety We take the existing doc-comment from rust-cpython and make it compile (validated by `cargo test`). With the added explanations, the ordinary comment was no longer useful, we could therefore remove it. The new explanation stresses that "not leaking the internal faked reference" is definitely not enough, because the problem is about *all references* that can be derived from it. We ended up duplicating the explanation, because that is a way to ensure that people do not miss it. Also, it was a bit misleading that the previous example was for `try_borrow_mut()`, so we made a similar, simpler one for `try_borrow()`.