update comment

Author: yaahc

compiler/rustc_passes/src/stability.rs

@@ -820,7 +820,10 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
             );
 
             if item_is_allowed {
-                // Check parent modules stability as well
+                // Check parent modules stability as well if the item the path refers to is itself
+                // stable. We only emit warnings for unstable path segments if the item is stable
+                // because stability is often inherited, so the most common case is that both the
+                // segments and the item are unstable behind the same feature flag.
                 //
                 // We check here rather than in `visit_path_segment` to prevent visiting the last
                 // path segment twice