Be more specific why JSON key names may use different naming policy

Signed-off-by: Pekka Helenius <fincer89@hotmail.com>

bookstore/src/main/resources/messages.properties

@@ -60,7 +60,7 @@ page.text.list.delete              = Delete
 page.text.list.edit                = Edit
 page.text.list.json                = Get
 
-page.text.apiref.warning           = NOTE\:\ these direct JSON keys may differ in name \&\ count when comparing to publicly exposed JSON book data.
+page.text.apiref.warning           = NOTE\:\ these direct JSON keys may differ in name \&\ count when comparing to publicly exposed JSON book data due to custom JSON serializers used in Java code.
 
 page.text.apiref.endpoints         = REST API end points
 page.text.apiref.examples          = Direct search examples