When rustdoc lints were changed to be tool lints, the `rustdoc` group
was removed, leading to spurious warnings like

```
warning: unknown lint: `rustdoc`
```

The lint group still worked when rustdoc ran, since rustdoc added the group itself.

This renames the group to `rustdoc::all` for consistency with
`clippy::all` and the rest of the rustdoc lints.