Clarify a vague comment

src/librustc_typeck/check/mod.rs

@@ -1394,7 +1394,8 @@ fn check_union(tcx: TyCtxt<'_>, id: hir::HirId, span: Span) {
 /// When the `#![feature(untagged_unions)]` gate is active,
 /// check that the fields of the `union` does not contain fields that need dropping.
 fn check_union_fields(tcx: TyCtxt<'_>, _: Span, item_def_id: DefId) -> bool {
-    // Without the feature we check Copy types only later
+    // Without the feature we check that all fields are `Copy` in our stability checking
+    // infrastructure.
     if !tcx.features().untagged_unions {
         return true;
     }