refactor(transformer): comment about potential improvement to arrow function transform (#5841)

crates/oxc_transformer/src/es2015/arrow_functions.rs

@@ -246,6 +246,10 @@ impl<'a> ArrowFunctions<'a> {
         span: Span,
         ctx: &mut TraverseCtx<'a>,
     ) -> IdentifierReference<'a> {
+        // TODO(improve-on-babel): We create a new UID for every scope. This is pointless, as only one
+        // `this` can be in scope at a time. We could create a single `_this` UID and reuse it in each
+        // scope. But this does not match output for some of Babel's test cases.
+        // <https://github.com/oxc-project/oxc/pull/5840>
         let this_var = self.this_var_stack.last_mut().unwrap();
         if this_var.is_none() {
             let target_scope_id =