Rename `rustdoc` to `rustdoc::all`
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.
Follow-up to #80527.
r? ``@Manishearth``
33614023a1