Change (P)Config::set return value to bool

src/Core/Config/IConfigAdapter.php

@@ -53,7 +53,7 @@ interface IConfigAdapter
 	 * @param string $key    The configuration key to set
 	 * @param mixed  $value  The value to store
 	 *
-	 * @return mixed Stored $value or false if the database update failed
+	 * @return bool Operation success
 	 */
 	public function set($cat, $k, $value);