EZP-21344: [doc]  updating documentation to reflect real behavior

Changing the response to 200 in the POST example, according to the process described in the document.
The HTTP response is 200 OK and not 201 Created, because the storage is not implemented yet.
Adding note on the future implementation : with storage/persistance, it will be 201 Created.
Merge pull request #772 from mere-teresa/EZP-21344

EZP-21344: [REST Spec] updating doc on HTTP response