save_code_now: Display list of supported tarball formats in help page

Give more details about what kind of tarballs can be loaded into the
archive through the save code now service.

swh/web/save_code_now/templates/origin-save-help.html

@@ -36,7 +36,9 @@ See top-level LICENSE file for more information
           {% endif %}
           {% if "tarball" in visit_types %}
             <li>
-              <code>tarball</code>, for tarball origins
+              <code>tarball</code>, for tarball origins (supported formats: <code>.jar</code>, <code>.tar</code>,
+              <code>.tar.bz2</code>,  <code>.tar.gz</code>, <code>.tar.lz</code>, <code>.tar.xz</code>,
+              <code>.tar.zst</code>, <code>.zip</code>)
             </li>
           {% endif %}
         </ul>