Updated comment for storeRelativeAgeDirective method

src/Symfony/Component/HttpKernel/HttpCache/ResponseCacheStrategy.php

@@ -194,7 +194,13 @@ private function willMakeFinalResponseUncacheable(Response $response)
     }
 
     /**
-     * Stores the cache directive relative to the age of the response.
+     * Store lowest max-age/s-maxage/expires for the final response.
+     *
+     * The response might have been stored in cache a while ago. To keep things comparable,
+     * we have to subtract the age so that the value is normalized for an age of 0.
+     *
+     * If the value is lower than the currently stored value, we update the value, to keep a rolling
+     * minimal value of each instruction.
      *
      * @param string   $cacheKey
      * @param int|null $value