Update 18-class.md

Rest of doc references 5.16 but this is from 5.19 so added the clarification.

documentation/docs/03-template-syntax/18-class.md

@@ -71,7 +71,7 @@ The user of this component has the same flexibility to use a mixture of objects,
 </Button>
 ```
 
-Svelte also exposes the `ClassValue` type, which is the type of value that the `class` attribute on elements accept. This is useful if you want to use a type-safe class name in component props:
+Since Svelte 5.19, Svelte also exposes the `ClassValue` type, which is the type of value that the `class` attribute on elements accept. This is useful if you want to use a type-safe class name in component props:
 
 ```svelte
 <script lang="ts">