Add messagecode resolver format based on application property

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -68,6 +68,7 @@ content into your application; rather pick only the properties that you need.
 	http.mappers.json-pretty-print=false # pretty print JSON
 	http.mappers.json-sort-keys=false # sort keys
 	spring.mvc.locale= # set fixed locale, e.g. en_UK
+	spring.mvc.message-codes-resolver.format= # PREFIX_ERROR_CODE / POSTFIX_ERROR_CODE
 	spring.view.prefix= # MVC view prefix
 	spring.view.suffix= # ... and suffix
 	spring.resources.cache-period= # cache timeouts in headers sent to browser