Clarify warning. Add comment.

lib/mixins/property-effects.js

@@ -2390,8 +2390,14 @@ export const PropertyEffects = dedupingMixin(superClass => {
      * @protected
      */
     static _addTemplatePropertyEffect(templateInfo, prop, effect) {
+      // `dynamicFns` is the flattened property list, so we can use that to
+      // detect non-declared properties.  Properties must be listed in
+      // `properties` to be included in `observedAttributes` since CE V1
+      // reads that at registration time, and we want to keep template parsing
+      // lazy
       if (legacyOptimizations && !(prop in templateInfo.dynamicFns)) {
-        console.warn(`Property '${prop}' used in template but not declared in 'properties'.`);
+        console.warn(`Property '${prop}' used in template but not declared in 'properties'; ` +
+          `attribute will not be observed.`);
       }
       let hostProps = templateInfo.hostProps = templateInfo.hostProps || {};
       hostProps[prop] = true;