add a fourth header

Author: Rich-Harris

documentation/docs/03-template-syntax/06-snippet.md

@@ -190,6 +190,8 @@ Any content inside the component tags that is _not_ a snippet declaration implic
 
 > [!NOTE] Note that you cannot have a prop called `children` if you also have content inside the component — for this reason, you should avoid having props with that name
 
+### Optional snippet props
+
 You can declare snippet props as being optional. You can either use optional chaining to not render anything if the snippet isn't set...
 
 ```svelte