`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 {
Some(<$T>::MIN)
}
```

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

Search the repository for RealField::min_value and the implementation shown in the issue. Read the surrounding trait or macro definitions to determine whether the documentation or the returned value is authoritative, then update the mismatched part and verify the relevant checks pass.

Written by the indexing model from the issue text.