Document i32 and f64 as copied values from R

Hi, actually when we use a function like:

```R
rust_function("fn add(a:f64, b:f64) -> f64 { a + b }")
```

The values ```a``` and ```b``` are copied, but if we interpret it from Rust, we would think the library moved the values from R to Rust, so they should not be available now.

An option would be document this behavior, other one, which I think is a lot more clear is use ```&i32``` and ```&f64```.

Thx!