Document $$props

This was explained in the tutorial, but I could not find any documentation on it in the docs.

site/content/docs/02-template-syntax.md

@@ -94,6 +94,12 @@ An element or component can have multiple spread attributes, interspersed with r
 <Widget {...things}/>
 ```
 
+*`$$props`* references all props that are passed to a component – including ones that are not declared with `export`. It is useful in rare cases, but not generally recommended, as it is difficult for Svelte to optimise.
+
+```html
+<Widget {...$$props}/>
+```
+
 
 ### Text expressions