templates/ident: add note about autocomplete to "Identifier not used"

Current autocomplete behaviour is unintuitive. It is a good idea to
explain to users why it can suggest invalid identifiers.

templates/ident.html

@@ -58,6 +58,7 @@
     {% endfor %}
 {% else %}
     <h2>Identifier not used</h2>
+    <div id="error-details">Note: Autocomplete currently searches identifiers in all versions of the project.</div>
 {% endif %}
 </div>