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.

compiler/rustc_lint/src/lib.rs

@@ -343,6 +343,7 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
         "intra_doc_link_resolution_failure",
         "use `rustdoc::broken_intra_doc_links` instead",
     );
+    store.register_removed("rustdoc", "use `rustdoc::all` instead");
 
     store.register_removed("unknown_features", "replaced by an error");
     store.register_removed("unsigned_negation", "replaced by negate_unsigned feature gate");