Update help text of /api-url

public/script.js

@@ -9107,6 +9107,10 @@ jQuery(async function () {
                 If a manual API is provided to <b>set</b>the URL, make sure to set <code>connect=false</code>, as auto-connect only works for the currently selected API,
                 or consider switching to it with <code>/api</code> first.
             </div>
+            <div>
+                This slash command works for most of the Text Completion sources, and also Custom OpenAI compatible for the Chat Completion sources. If unsure which APIs are supported,
+                check the auto-completion of the optional <code>api</code> argument of this command.
+            </div>
         `,
     }));
     SlashCommandParser.addCommandObject(SlashCommand.fromProps({