clarify some comments

compiler/noirc_frontend/src/hir/type_check/expr.rs

@@ -857,7 +857,9 @@ impl<'interner> TypeChecker<'interner> {
 
                 for constraint in func_meta.trait_constraints {
                     if let Some(trait_id) = constraint.trait_id {
-                        // TODO: this == check *seems* to work?
+                        // TODO: == on types is sketchy, because Field != TypeVar::Bound(Field)
+                        // unify() is sketchier here though, since it may accidentally commit typebindings.
+                        // this works for now, but probably needs to be revisited when implementing generic traits
                         if *object_type == constraint.typ {
                             let the_trait = self.interner.get_trait(trait_id);
                             let the_trait = the_trait.borrow();

compiler/noirc_frontend/src/monomorphization/mod.rs

@@ -787,6 +787,9 @@ impl<'interner> Monomorphizer<'interner> {
     ) -> ast::Expression {
         let function_type = self.interner.id_type(expr_id);
 
+        // the substitute() here is to replace all internal occurences of the 'Self' typevar
+        // with whatever 'Self' is currently bound to, so we don't lose type information
+        // if we need to rebind the trait.
         let trait_impl = self
             .interner
             .get_trait_implementation(&TraitImplKey {