Undocumented breaking change

Going from 10 to 11, or 10 to 12.

```diff
-import { parseDOM } from 'htmlparser2';
+import { parseDocument } from 'htmlparser2';

 function parseMarkup (markup) {
   const options = {};
-  const ast = parseDOM(markup, options);
+  const ast = parseDocument(markup, options).children;
   return ast;
 };
```
