docs for Update activities

templates/docs.html

@@ -44,6 +44,7 @@ Bridgy Fed takes some technical know-how to set up, and there are simpler (but l
 <li><a href="#like">How do I favorite (aka like) or boost (aka repost) a fediverse post?</a></li>
 <li><a href="#follow">How do I follow someone?</a></li>
 <li><a href="#image">How do I include an image in a post?</a></li>
+<li><a href="#update">How do I edit an existing post?</a></li>
 <li><a href="#fragment">Can I publish just one part of a page?</a></li>
 <li><a href="#backfeed">How do fediverse replies, likes, and other interactions show up on my site?</a></li>
 <li><a href="#read">How do I read my fediverse timeline/feed?</a></li>
@@ -285,6 +286,12 @@ I love scotch. Scotchy scotchy scotch.
 </p>
 </li>
 
+<li id="update" class="question">How do I edit an existing post?</li>
+<li class="answer">
+<p>Edit the post on your web site, then send another webmention to Bridgy Fed for it. Bridgy Fed will refetch the post and send an <a href="https://www.w3.org/TR/activitypub/#update-activity-outbox"><code>Update</code> activity</a> for it to the fediverse.
+</p>
+</li>
+
 <li id="fragment" class="question">Can I publish just one part of a page?</li>
 <li class="answer">
 <p>If that HTML element has its own id, then sure! Just put the id in the fragment of the URL that you publish. For example, to publish the <code>bar</code> post here:</p>