Docs for oneOfInputUnion: true

### Is your feature request related to a problem? Please describe.

I've observed that `oneOfInputUnion: true` will change the code that gets generated.

```graphql
input YinOrYangInput @oneOf {
yin: YinConfig
yang: YangConfig
}
```

If you have this GraphQL schema, ordinarily you'll get the TS version:

```ts
export type YinOrYangInput = {
yin?: Maybe;
yang?: Maybe;
}
```

🔍👻 But if you add this mysterious `oneOfInputUnion: true` config, then the generated output changes to a union type:

```ts
export type YinOrYangInput =
| { yin: YinConfig, yang?: never }
| { yin?: never, yang: YangConfig }
```

### Describe the solution you'd like

From what I can tell, documentation for this config option is non-existent.

### Describe alternatives you've considered

_No response_

### Any additional important details?

_No response_