bootstrap: add PhpDoc to critical_error to make phpstan happy

it wants phpdoc or typehints, and we can't provide typehints because bootstrap needs to be compatible with old PHP versions.

src/pocketmine/PocketMine.php

@@ -36,6 +36,10 @@ namespace pocketmine {
 
 	const MIN_PHP_VERSION = "7.2.0";
 
+	/**
+	 * @param string $message
+	 * @return void
+	 */
 	function critical_error($message){
 		echo "[ERROR] $message" . PHP_EOL;
 	}