Improve Kotlin ref doc
Improve Kotlin ref doc

This commit add a section about annotations and provides
various update and enhancements to the Kotlin reference
documentation.

Issue: SPR-15659

spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework