Rollup merge of #120699 - nnethercote:rm-useless-TRACK_DIAGNOSTIC-calls, r=oli-obk

Document `TRACK_DIAGNOSTIC` calls.

r? ```````@cjgillot```````

compiler/rustc_errors/src/emitter.rs

@@ -2086,7 +2086,7 @@ impl HumanEmitter {
                 }
                 if !self.short_message {
                     for child in children {
-                        assert!(child.level.can_be_top_or_sub().1);
+                        assert!(child.level.can_be_subdiag());
                         let span = &child.span;
                         if let Err(err) = self.emit_messages_default_inner(
                             span,