Clarify "downloadable" (#324)

Make it clear that resource timing reflects fetches of HTTP(s) resources,
which is a more accurate term than "downloadable".

index.html

@@ -237,12 +237,12 @@ <h3>
         </h3>
         <p data-fn-for="html">
           The <a>PerformanceResourceTiming</a> interface facilitates timing
-          measurement of downloadable resources. For example, this interface is
-          available for {{XMLHttpRequest}} objects [[XHR]], HTML elements
+          measurement of [=fetch|fetched=] [=http(s) scheme|http(s)=] resources. For example,
+          this interface is available for {{XMLHttpRequest}} objects [[XHR]], HTML elements
           [[HTML]] such as [^iframe^], [^img^], [^script^], [^object^],
           [^embed^] and [^link^] with the link type of [^link/rel/stylesheet^],
-          and SVG elements [[SVG11]] such as <a data-cite=
-          "SVG11/struct.html#SVGElement">svg</a>.
+          SVG elements [[SVG11]] such as <a data-cite=
+          "SVG11/struct.html#SVGElement">svg</a>, and {{EventSource}}.
         </p>
       </section>
       <section>