Refine comments for get_stale_cache method in WP 404 cache class

Improved the code comments for the get_stale_cache method in the class-full-page-cache-404.php file. The changes provide a more comprehensive explanation of the method's return values.

src/features/class-full-page-cache-404.php

@@ -274,7 +274,7 @@ public static function get_cache(): mixed {
 	/**
 	 * Get stale cache.
 	 *
-	 * @return mixed
+	 * @return mixed The cache contents on success, false on failure to retrieve contents.
 	 */
 	public static function get_stale_cache(): mixed {
 		return wp_cache_get( self::STALE_CACHE_KEY, self::CACHE_GROUP );