Update documentation for heading attributes

Author: ehuss

guide/src/format/markdown.md

@@ -221,7 +221,7 @@ To enable it, see the [`output.html.curly-quotes`] config option.
 [task list extension]: https://github.github.com/gfm/#task-list-items-extension-
 [`output.html.curly-quotes`]: configuration/renderers.md#html-renderer-options
 
-### Heading attributes { #headingattributes .class1 .class2 }
+### Heading attributes
 
 Headings can have a custom HTML ID and classes. This let's you maintain the same ID even if you change the heading's text, it also let's you add multiple classes in the heading.
 
@@ -231,3 +231,5 @@ Example:
 ```
 
 This makes the level 1 heading with the content `Example heading`, ID `first`, and classes `class1` and `class2`. Note that the attributes should be space-separated.
+
+More information can be found in the [heading attrs spec page](https://github.com/raphlinus/pulldown-cmark/blob/master/specs/heading_attrs.txt).