fix(storage): storage format

Fix on the storage format (probably need to be reworded).

Author: Guikingone

workflow/usage.rst

@@ -166,6 +166,13 @@ As configured, the following property is used by the marking store::
 
 .. note::
 
+    If a ``multiple_state`` is used, the persisted state type should be an ``array``,
+    using Doctrine, you can use ``json`` or ``json_array``.
+
+    If a ``single_state`` is used, the persisted state type could be a string
+
+    If something else is used, you're in charge of choosing the best storage format.
+    
     Please note that if you decide to store $currentPlace using Doctrine,
     the attribute must be mapped to an array.