Add docBlock and fix CS

src/Composer/Json/JsonFile.php

@@ -215,6 +215,12 @@ class JsonFile
         return $result;
     }
 
+    /**
+     * Throws an exception according to a given code with a customized message
+     *
+     * @param int $code return code of json_last_error function
+     * @throws \RuntimeException
+     */
     private static function throwEncodeError($code)
     {
         switch ($code) {