docs: document rune_missing_parentheses compiler error (#18589)

documentation/docs/98-reference/.generated/compile-errors.md

@@ -821,6 +821,18 @@ Cannot use `%rune%` rune in non-runes mode
 Cannot use rune without parentheses
 ```
 
+Runes are keywords rather than values — they can't be assigned to a variable or passed to a function, only called. Referencing one without parentheses is therefore an error...
+
+```js
+let count = $state;
+```
+
+...whether it's a rune like `$state` or one reached through a property, like `$derived.by`. Add the parentheses, along with any arguments the rune expects:
+
+```js
+let count = $state(0);
+```
+
 ### rune_removed
 
 ```

packages/svelte/messages/compile-errors/script.md

@@ -186,6 +186,18 @@ This turned out to be buggy and unpredictable, particularly when working with de
 
 > Cannot use rune without parentheses
 
+Runes are keywords rather than values — they can't be assigned to a variable or passed to a function, only called. Referencing one without parentheses is therefore an error...
+
+```js
+let count = $state;
+```
+
+...whether it's a rune like `$state` or one reached through a property, like `$derived.by`. Add the parentheses, along with any arguments the rune expects:
+
+```js
+let count = $state(0);
+```
+
 ## rune_removed
 
 > The `%name%` rune has been removed