Document that all-hosts works as an optional permission.

chrome/common/extensions/docs/templates/intros/permissions.html

 <h2 id="howto"> Implementing optional permissions </h2>
 
 <h3 id="types">
 Step 1: Decide which permissions are required and which are optional
 </h3>
 
 <p>
 An extension can declare both required and optional permissions. In general, you should:
 <ul>
   <li>Use required permissions when they are needed for your extension’s basic functionality.</li>
@@ skipping 59 matching lines @@
   <li>notifications</li>
   <li>pageCapture</li>
   <li>tabs</li>
   <li>topSites</li>
   <li>webNavigation</li>
   <li>webRequest</li>
   <li>webRequestBlocking</li>
 </ul>
 </p>
 
+<p>If you want to request hosts that you only discover at runtime, include
+<code>"http://*/"</code> and/or <code>"https://*/"</code> as an
+<code>optional_permission</code>.  This lets you specify any origin in
+$ref:Permissions.origins as long as it has a matching scheme.
+</p>
+
 <h3 id="request"> Step 3: Request optional permissions </h3>
 <p>
   Request the permissions from within a user gesture using
   <code>permissions.request()</code>:
 <pre>
 document.querySelector('#my-button').addEventListener('click', function(event) {
   // Permissions must be requested from inside a user gesture, like a button's
   // click handler.
   chrome.permissions.request({
     permissions: ['tabs'],
@@ skipping 56 matching lines @@
   origins: ['http://www.google.com/']
 }, function(removed) {
   if (removed) {
     // The permissions have been removed.
   } else {
     // The permissions have not been removed (e.g., you tried to remove
     // required permissions).
   }
 });
 </pre>