Make the Background section of Processing model non-normative.

This fixes issue WebAudio#1903.

index.bs

@@ -10601,18 +10601,20 @@ registerProcessor('VUMeter', class extends AudioWorkletProcessor {
 <h2 id="processing-model">
 Processing model</h2>
 
-<h3 id="processing-model-background">
+<h3 id="processing-model-background" class="non-normative">
 Background</h3>
 
+<em>This section is non-normative.</em>
+
 Real-time audio systems that require low latency are often
 implemented using <em>callback functions</em>, where the operating
 system calls the program back when more audio has to be computed in
 order for the playback to stay uninterrupted. Such a callback is ideally called
 on a high priority thread (often the highest priority on the system).
 This means that a program that deals with audio only executes code
-from this callback, as any buffering between a rendering thread and
-the callback would naturally add latency or make the system less
-resilient to glitches.
+from this callback. Crossing thread boundaries or adding some buffering between
+a rendering thread and the callback would naturally add latency or make the
+system less resilient to glitches.
 
 For this reason, the traditional way of executing asynchronous
 operations on the Web Platform, the event loop, does not work here,