Add note about additional operations when preloading

Closes w3c/preload#116.

source

@@ -25768,6 +25768,11 @@ document.body.appendChild(wbr);</code></pre>
   data-x="concept-request-destination">destination</span>), as it is highly likely that the user
   will require this resource for the current navigation. <ref spec=PRELOAD></p>
 
+  <p class="note">User-agents might perform additional operations when a resource is loaded, such as
+  preemptively <span data-x="dom-img-decode">decoding images</span> or <span
+  data-x="Create a CSS style sheet">creating stylesheets</span>. However, these additional
+  operations cannot have observable effects.</p>
+
   <p>There is no default type for resources given by the <code data-x="rel-preload">preload</code>
   keyword.</p>