Mention that enums must be backed enums

Author: javiereguiluz

reference/forms/types/enum.rst

@@ -30,7 +30,8 @@ Example Usage
 -------------
 
 Before using this field, you'll need to have some PHP enumeration (or "enum" for
-short) defined somewhere in your application::
+short) defined somewhere in your application. This enum has to be of type
+"backed enum", where each keyword defines a scalar value such a string::
 
     // src/Config/TextAlign.php
     namespace App\Config;