update docs for `$$props` and `$$restProps`

Adds example of case where `$$props` can be useful. Adds similar optimization warning as `$$props` to `$$restProps`

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

@@ -107,7 +107,7 @@ An element or component can have multiple spread attributes, interspersed with r
 
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-*`$$props`* references all props that are passed to a component – including ones that are not declared with `export`. It is useful in rare cases, but not generally recommended, as it is difficult for Svelte to optimise.
+*`$$props`* references all props that are passed to a component, including ones that are not declared with `export`. It is not generally recommended, as it is difficult for Svelte to optimise. But it can be useful in rare cases – for example, when you don't know at compile time what props might be passed to a component.
 
 ```sv
 <Widget {...$$props}/>
@@ -115,7 +115,7 @@ An element or component can have multiple spread attributes, interspersed with r
 
 ---
 
-*`$$restProps`* contains only the props which are *not* declared with `export`. It can be used to pass down other unknown attributes to an element in a component.
+*`$$restProps`* contains only the props which are *not* declared with `export`. It can be used to pass down other unknown attributes to an element in a component. It shares the same optimisation problems as *`$$props`*, and is likewise not recommended.
 
 ```html
 <input {...$$restProps}>