`RealField::min_value()` documentation does not match implementation

Hi,

`min_value()` is implemented as 

```rust
/// The smallest finite positive value representable using this type.
#[inline]
fn min_value() -> Option<Self> {
    Some(<$T>::MIN)
}
```

But `MIN` is negative. `MIN_POSITIVE` might be the intended implementation if documentation is correct.