Document that Jackson is a required dependency of the Actuator

Closes gh-19564

Author: snicoll

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/asciidoc/index.adoc

@@ -20,7 +20,7 @@ Before you proceed, you should read the following topics:
 * <<overview-endpoint-urls>>
 * <<overview-timestamps>>
 
-
+NOTE: In order to get the correct JSON responses documented below, Jackson must be available.
 
 [[overview-endpoint-urls]]
 === URLs

spring-boot-project/spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -976,6 +976,8 @@ For example, `health` is exposed as `/actuator/health`.
 TIP: Actuator is supported natively with Spring MVC, Spring WebFlux, and Jersey.
 If both Jersey and Spring MVC are available, Spring MVC will be used.
 
+NOTE: Jackson is a required dependency in order to get the correct JSON responses as documented in the API documentation ({spring-boot-actuator-restapi}/html[HTML] or {spring-boot-actuator-restapi}/pdf/spring-boot-actuator-web-api.pdf[PDF]).
+
 
 
 [[production-ready-customizing-management-server-context-path]]