a ton of improvements

docs/src/demos/Extensions/Heading/index.vue

@@ -44,10 +44,10 @@ export default {
         }),
       ],
       content: `
-        <h1>This is a headline level 1</h1>
-        <h2>This is a headline level 2</h2>
-        <h3>This is a headline level 3</h3>
-        <h4>This headline will be converted to a paragraph, because it's not defined in the levels option.</h4>
+        <h1>This is a 1st level heading</h1>
+        <h2>This is a 2nd level heading</h2>
+        <h3>This is a 3rd level heading</h3>
+        <h4>This 4th level heading will be converted to a paragraph, because levels are configured to be only 1, 2 or 3.</h4>
       `,
     })
   },