Add todo comment

compiler/rustc_session/src/parse.rs

@@ -77,6 +77,7 @@ pub fn insert(&self, symbol: Symbol, span: Span) {
     }
 }
 
+// todo: this function now accepts `Session` instead of `ParseSess` and should be relocated
 /// Construct a diagnostic for a language feature error due to the given `span`.
 /// The `feature`'s `Symbol` is the one you used in `unstable.rs` and `rustc_span::symbols`.
 #[track_caller]