Document health aggregation strategy and default status mappings

Closes gh-9580

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

@@ -283,8 +283,10 @@ returned, and for an authenticated connection additional details are also displa
 Health information is collected from all
 {sc-spring-boot-actuator}/health/HealthIndicator.{sc-ext}[`HealthIndicator`] beans defined
 in your `ApplicationContext`. Spring Boot includes a number of auto-configured
-`HealthIndicators` and you can also write your own.
-
+`HealthIndicators` and you can also write your own. By default, the final system state is derived
+by the `HealthAggregator` which sorts the statuses from each `HealthIndicator` based on an ordered list of statuses.
+The first status in the sorted list is used as the overall health status.
+If no `HealthIndicator` returns a status that is known to the `HealthAggregator`, an `UNKNOWN` status is used.
 
 
 === Security with HealthIndicators
@@ -400,6 +402,25 @@ For example, the following maps `FATAL` to `HttpStatus.SERVICE_UNAVAILABLE`:
 	endpoints.health.mapping.FATAL=503
 ----
 
+The default status mappings for the built-in statuses are:
+
+[cols="1,3"]
+|===
+|Status |Mapping
+
+|DOWN
+|SERVICE_UNAVAILABLE (503)
+
+|OUT_OF_SERVICE
+|SERVICE_UNAVAILABLE (503)
+
+|UP
+|No mapping by default, so http status is 200
+
+|UNKNOWN
+|No mapping by default, so http status is 200
+|===
+
 
 
 [[production-ready-application-info]]