Document the scrollpastend addon

Closes codemirror#6381

Author: marijnh

doc/manual.html

@@ -3147,6 +3147,11 @@ <h2 id="addons">Addons</h2>
         A demo of the addon is available <a href="../demo/hardwrap.html">here</a>.
       </dd>
 
+      <dt id="addon_scrollpastend"><a href="../addon/scroll/scrollpastend.js"><code>scroll/scrollpastend.js</code></a></dt>
+      <dd>Defines an option `"scrollPastEnd"` that, when set to a
+      truthy value, allows the user to scroll one editor height of
+      empty space into view at the bottom of the editor.</dd>
+
       <dt id="addon_merge"><a href="../addon/merge/merge.js"><code>merge/merge.js</code></a></dt>
       <dd>Implements an interface for merging changes, using either a
       2-way or a 3-way view. The <code>CodeMirror.MergeView</code>