fix: add note about non-standard property attribute

Author: evanp

index.html

@@ -925,7 +925,7 @@ <h5>Example</h5>
             &lt;/html&gt;
           </pre>
           <p>
-            In this example, the <code>article:author</code> property is the author of the document.
+            In this example, the <code>article:author</code> property is the author of the document. (Note that OpenGraph Protocol uses the non-standard <code>property</code> attribute for metadata.)
           </p>
           <p>
             Given an HTML page at <code>https://html.example/files/video-40.html</code> that represents a <code>Video</code> object, the consumer can use OpenGraph metadata to identify the Webfinger ID of the creator of the video: