docs: Improve Quick start instructions (#1466)

Co-authored-by: Luffy <[email protected]>

Author: waldyrious

docs/quickstart.md

@@ -124,7 +124,7 @@ If you want, you can show a loading dialog before docsify starts to render your
 <div id="app">Please wait...</div>
 ```
 
-You should set the `data-app` attribute if you changed `el`:
+By default, the `id` attribute of the main container is `app`. If you want to use a different id, you must also set the `data-app` attribute, and [configure](configuration.md#el) the `el` parameter accordingly:
 
 ```html
 <!-- index.html -->
@@ -138,8 +138,6 @@ You should set the `data-app` attribute if you changed `el`:
 </script>
 ```
 
-Compare [el configuration](configuration#el).
-
 <script>
   (function() {
     const linkElm = document.querySelector('#template a[download="index.html"]');