[skip ci] doc fixes

Author: Steven Orvell

lib/mixins/property-accessors.html

@@ -477,11 +477,12 @@
       }
 
       /**
-       * Call to enable property accessors. This method must be called
-       * for any side effects of setting properties to occur. For elements,
-       * generally `connectedCallback` is a normal spot to do so.
-       * It is safe to call this method multiple times as it only turns
-       * on property accessors once.
+       * Call to enable property accessor processing. Before this method is
+       * called accessor values will be set but side effects are
+       * queued. When called, any pending side effects occur immediately.
+       * For elements, generally `connectedCallback` is a normal spot to do so.
+       * It is safe to call this method multiple times as it only turns on
+       * property accessors once.
        */
       _enableProperties() {
         if (!this.__dataEnabled) {

lib/mixins/property-effects.html

@@ -1432,6 +1432,9 @@
         if (!this.__dataClientsInitialized) {
           this.__dataClientsInitialized = true;
           this._readyClients();
+          // Override point where accessors are turned on; importantly,
+          // this is after clients have fully readied, providing a guarantee
+          // that any property effects occur only after all clients are ready.
           this.__dataInitialized = true;
         } else {
           // Flush all clients