[docs] `ptr::eq` source does not mention smart-pointer comparison behavior.

The documentation at the [`ptr::eq` source] doesn't mention that smart pointers are compared in their totality, unlike pointer comparison which just compares the addresses. I found out about this behavior on [this urlo question](https://users.rust-lang.org/t/shallow-compare-for-slices/26321/3).

[`ptr::eq` source]: https://github.com/rust-lang/rust/blob/master/src/libcore/ptr.rs#L2476