Really mark class `Icinga\Web\Hook` deprecated -.-

I would have removed it, but the deprecation notice wasn't
defined properly and so not visible for more than six years.
Now even our own projects make still use of it thoroughly..

library/Icinga/Web/Hook.php

@@ -8,7 +8,8 @@ use Icinga\Application\Hook as NewHookImplementation;
 /**
  * Icinga Web Hook registry
  *
- * Deprecated, please use Icinga\Application\Hook instead
+ * @deprecated It is highly recommended to use {@see Icinga\Application\Hook} instead. Though since this message
+ *             (or rather the previous message) hasn't been visible for ages... This won't be removed anyway....
  */
 class Hook extends NewHookImplementation
 {