docs: Add JSDoc comment to type guard function (#1596)

Author: garmeeh

.changeset/tiny-docs-improvement.md

@@ -0,0 +1,5 @@
+---
+"next-seo": patch
+---
+
+Add JSDoc comment to internal type guard function

src/utils/stringify.ts

@@ -38,7 +38,10 @@ const safeJsonLdReplacer: JsonReplacer = (() => {
   };
 })();
 
-// Utility: Assert never
+/**
+ * Type guard to ensure exhaustive type checking.
+ * @internal
+ */
 function isNever(_: never): void {}
 
 /**