Update "Observing event listeners" section

Addesses @annevk's feedback here: WICG/EventListenerOptions#20 (comment)

dom.bs

@@ -1135,21 +1135,22 @@ method must run these steps:
 </ol>
 
 <h3 id=observing-event-listeners>Observing event listeners</h3>
-This section is non-normative.
-
-In general, the mere presence of an event listener should not result in any
-behavior change.  Developers don't expect an event listener to have any impact
-on behavior other than that done explicitly by their callback.
-
-However, in some cases a user agent may make important performance optimizations
-in scenarios where there are no event listeners of a given type (such as by leaving
-a sensor disabled when there are no consumers for the data).  In rare cases, a
-particular type of event may be specified to be cancelable only when there is at
-least one non-passive listener.  For example, non-passive touch event listeners
-must blocking scrolling, but if all listeners are passive then a performance
-optimization is possible by making the event uncancelable.  New event types
-should be defined such that they don't need this property (use
-<a href="https://lists.w3.org/Archives/Public/public-script-coord/">public-scrip-coord</a>
+In general, developers don't expect the presence of an event listener to be
+observable.  The impact of an event listener is determined by it's callback.
+That is, a developer adding a no-op event listener would not expect it to have
+any side effects.
+
+Unfortunately, some event APIs have been designed such that implementing them
+efficiently requires observing event listeners.  For example, sensor APIs which
+enable an underlying device sensor, and touch APIs which can be used to block
+async scrolling.  In some cases this problem can be mitigated by specifying
+the event to be {{Event/cancelable}} only when there is at least one
+non-{{EventListenerOptions/passive}} listener.  For example, non-passive
+{{TouchEvent}} listeners must block scrolling, but if all listeners are passive then
+scrolling can be allowed to start asynchronously by making the {{TouchEvent}}
+uncancelable (so that calls to {{Event/preventDefault()}} are ignored).
+New event types should be defined such that they don't need this
+property (use <a href="https://lists.w3.org/Archives/Public/public-script-coord/">public-scrip-coord@w3.org</a>
 for discussion).
 
 <h3 id=dispatching-events>Dispatching events</h3>

dom.html

@@ -69,7 +69,7 @@
   <div class="head">
    <p data-fill-with="logo"><a class="logo" href="https://whatwg.org/"> <img alt="WHATWG" height="100" src="https://resources.whatwg.org/logo-dom.svg"> </a> </p>
    <h1 class="p-name no-ref allcaps" id="title">DOM</h1>
-   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Living Standard — Last Updated <time class="dt-updated" datetime="2015-09-24">24 September 2015</time></span></h2>
+   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Living Standard — Last Updated <time class="dt-updated" datetime="2015-09-25">25 September 2015</time></span></h2>
    <div data-fill-with="spec-metadata">
     <dl>
      <dt>Participate:
@@ -767,18 +767,19 @@ <h3 class="heading settled" data-level="3.7" id="interface-eventtarget"><span cl
     <li><a data-link-type="dfn" href="#concept-event-dispatch">Dispatch</a> the <var>event</var> and return the value that returns. 
    </ol>
    <h3 class="heading settled" data-level="3.8" id="observing-event-listeners"><span class="secno">3.8. </span><span class="content">Observing event listeners</span><a class="self-link" href="#observing-event-listeners"></a></h3>
-    This section is non-normative. 
-   <p>In general, the mere presence of an event listener should not result in any
-behavior change.  Developers don’t expect an event listener to have any impact
-on behavior other than that done explicitly by their callback.</p>
-   <p>However, in some cases a user agent may make important performance optimizations
-in scenarios where there are no event listeners of a given type (such as by leaving
-a sensor disabled when there are no consumers for the data).  In rare cases, a
-particular type of event may be specified to be cancelable only when there is at
-least one non-passive listener.  For example, non-passive touch event listeners
-must blocking scrolling, but if all listeners are passive then a performance
-optimization is possible by making the event uncancelable.  New event types
-should be defined such that they don’t need this property (use <a href="https://lists.w3.org/Archives/Public/public-script-coord/">public-scrip-coord</a> for discussion).</p>
+    In general, developers don’t expect the presence of an event listener to be
+observable.  The impact of an event listener is determined by it’s callback.
+That is, a developer adding a no-op event listener would not expect it to have
+any side effects. 
+   <p>Unfortunately, some event APIs have been designed such that implementing them
+efficiently requires observing event listeners.  For example, sensor APIs which
+enable an underlying device sensor, and touch APIs which can be used to block
+async scrolling.  In some cases this problem can be mitigated by specifying
+the event to be <code class="idl"><a data-link-type="idl" href="#dom-event-cancelable">cancelable</a></code> only when there is at least one
+non-<code class="idl"><a data-link-type="idl" href="#dom-eventlisteneroptions-passive">passive</a></code> listener.  For example, non-passive <code class="idl"><a data-link-type="idl" href="http://www.w3.org/TR/touch-events/#touchevent-interface">TouchEvent</a></code> listeners must block scrolling, but if all listeners are passive then
+scrolling can be allowed to start asynchronously by making the <code class="idl"><a data-link-type="idl" href="http://www.w3.org/TR/touch-events/#touchevent-interface">TouchEvent</a></code> uncancelable (so that calls to <code class="idl"><a data-link-type="idl" href="#dom-event-preventdefault">preventDefault()</a></code> are ignored).
+New event types should be defined such that they don’t need this
+property (use <a href="https://lists.w3.org/Archives/Public/public-script-coord/">public-scrip-coord@w3.org</a> for discussion).</p>
    <h3 class="heading settled" data-level="3.9" id="dispatching-events"><span class="secno">3.9. </span><span class="content">Dispatching events</span><a class="self-link" href="#dispatching-events"></a></h3>
    <p>To <dfn data-dfn-for="Event" data-dfn-type="dfn" data-export="" id="concept-event-dispatch">dispatch<a class="self-link" href="#concept-event-dispatch"></a></dfn> an <a data-link-type="dfn" href="#concept-event">event</a> to a
 given object, with an optional <var>target override</var>, run these steps:</p>