Document Actuator's ObjectMapper isolation

Closes gh-47039

Author: wilkinsona

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/actuator/endpoints.adoc

@@ -383,6 +383,17 @@ management:
 TIP: See javadoc:org.springframework.boot.actuate.autoconfigure.endpoint.web.CorsEndpointProperties[] for a complete list of options.
 
 
+[[actuator.endpoints.json]]
+== JSON
+
+When working with JSON, Jackson is used for serialization and deserialization.
+By default, an isolated javadoc:com.fasterxml.jackson.databind.ObjectMapper[] is used.
+This isolation means that it does not share the same configuration as the application's `ObjectMapper` and it is not affected by `spring.jackson.*` properties.
+To disable this behavior and configure Actuator to use the application's `ObjectMapper`, set configprop:management.endpoints.jackson.isolated-object-mapper[] to `false`.
+Alternatively, you can define your own javadoc:org.springframework.boot.actuate.endpoint.jackson.EndpointObjectMapper[] bean that produces an `ObjectMapper` that meets your needs.
+Actuator will then use it for JSON processing.
+
+
 
 [[actuator.endpoints.implementing-custom]]
 == Implementing Custom Endpoints