Clarify ETag expiry return contract

Author: GrahamCampbell

src/EtagCachePlugin.php

@@ -41,6 +41,9 @@ public static function serverCache(CacheItemPoolInterface $pool, StreamFactoryIn
         return new self($pool, $streamFactory, $config);
     }
 
+    /**
+     * @return int
+     */
     protected function calculateResponseExpiresAt(?int $maxAge)
     {
         return 0;