`#[strum_discriminants(doc(hidden))]` doesn't work

I would like to generate discriminants for an `enum`, but hide them with `doc(hidden)`.  Unfortunately, the macro can't parse anything other than `doc = ...`:

```
error: expected `=`
   --> fidget-core/src/compiler/op.rs:293:30
    |
293 |     #[strum_discriminants(doc(hidden))]
    |      
```