Lint potential uses of `non_exhaustive`

Once rust-lang/rust#44109 merges, instances like this:

```
struct Test {
    pub val: u32,
    _priv: (),
}
```

Can be better worded as:

```
#[non_exhaustive]
struct Test {
    pub val: u32,
}
```

Additionally, having a unit variant at the end of an enum which is `doc(hidden)` should be marked as well.