add empty list response to API doc

res/template/fragments/api/list-post.html

@@ -71,6 +71,13 @@
 	"value": "description_too_long",
 	"message": "The description of this list or one of the files in the list is too long, max 3000 characters allowed."
 }
+</pre>
+<pre>HTTP 422: Unprocessable Entity
+{
+	"success": false,
+	"value": "cannot_create_empty_list",
+	"message": "You cannot make a list with no files."
+}
 </pre>
 	</div>
 </details>