mention const

Author: Rich-Harris

documentation/docs/02-runes/03-$derived.md

@@ -54,7 +54,7 @@ To exempt a piece of state from being treated as a dependency, use [`untrack`](s
 
 ## Overriding derived values
 
-Derived expressions are recalculated when their dependencies change, but you can temporarily override their values by reassigning them. This can be useful for things like _optimistic UI_, where a value is derived from the 'source of truth' (such as data from your server) but you'd like to show immediate feedback to the user:
+Derived expressions are recalculated when their dependencies change, but you can temporarily override their values by reassigning them (unless they are declared with `const`). This can be useful for things like _optimistic UI_, where a value is derived from the 'source of truth' (such as data from your server) but you'd like to show immediate feedback to the user:
 
 ```svelte
 <script>