Add comment for rustdoc overrides.

Author: cjgillot

Diff for: src/librustdoc/core.rs

@@ -269,6 +269,7 @@ pub(crate) fn create_config(
         parse_sess_created: None,
         register_lints: Some(Box::new(crate::lint::register_lints)),
         override_queries: Some(|_sess, providers, _external_providers| {
+            // We do not register late module lints, so this only runs `MissingDoc`.
             // Most lints will require typechecking, so just don't run them.
             providers.lint_mod = |tcx, module_def_id| late_lint_mod(tcx, module_def_id, MissingDoc);
             // hack so that `used_trait_imports` won't try to call typeck