Explain when rel="manifest" links should be fetched for installed apps

See discussion in w3c/manifest#1011 (comment).

source

@@ -25037,11 +25037,29 @@ document.body.appendChild(wbr);</code></pre>
   <p>There is no default type for resources given by the <code data-x="rel-manifest">manifest</code>
   keyword.</p>
 
-  <p>The appropriate time to <span>fetch and process the linked resource</span> for this link type
-  is when the user agent deems it necessary. For example, when the user chooses to <span
-  data-x="installed web application">install the web application</span>. In that case, only the
-  first <code>link</code> element in <span>tree order</span> whose <code
-  data-x="attr-link-rel">rel</code> attribute contains the token <code
+  <p>When a web application is not <span data-x="installed web application">installed</span>, the
+  appropriate time to <span>fetch and process the linked resource</span> for this link type is when
+  the user agent deems it necessary. For example, when the user chooses to <span data-x="installed
+  web application">install the web application</span>.</p>
+
+  <p>For an <span data-x="installed web application">installed web application</span>, the
+  appropriate times to <span>fetch and process the linked resource</span> for this link type
+  are:</p>
+
+  <ul>
+   <li><p>When the <span>external resource link</span> is created on a <code>link</code> element
+   that is already <span>browsing-context connected</span>.</p></li>
+
+   <li><p>When the <span>external resource link</span>'s <code>link</code> element <span>becomes
+   browsing-context connected</span>.</p></li>
+
+   <li><p>When the <code data-x="attr-link-href">href</code> attribute of the <code>link</code>
+   element of an <span>external resource link</span> that is already <span>browsing-context
+   connected</span> is changed.</p></li>
+  </ul>
+
+  <p>In either cases, only the first <code>link</code> element in <span>tree order</span> whose
+  <code data-x="attr-link-rel">rel</code> attribute contains the token <code
   data-x="rel-manifest">manifest</code> may be used.</p>
 
   <p>A user agent must not <span>delay the load event</span> for this link type.</p>