Clarify usage of BufferingApplicationStartup

See gh-25075

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

@@ -124,6 +124,7 @@ The following technology-agnostic endpoints are available:
 | `startup`
 | Shows the startup steps data collected by the `ApplicationStartup`.
   Requires the `SpringApplication` to be configured with a `BufferingApplicationStartup`.
+  More details about the implementations can be found at <<boot-features.adoc#boot-features-application-startup-tracking, application startup tracking>> page.
 
 | `threaddump`
 | Performs a thread dump.

spring-boot-project/spring-boot-docs/src/docs/asciidoc/spring-boot-features.adoc

@@ -486,6 +486,7 @@ Once configured, you can record data by running the application with the Flight
 Spring Boot ships with the `BufferingApplicationStartup` variant; this implementation is meant for buffering the startup steps and draining them into an external metrics system.
 Applications can ask for the bean of type `BufferingApplicationStartup` in any component.
 Additionally, Spring Boot Actuator will <<production-ready-features.adoc#production-ready-endpoints, expose a `startup` endpoint to expose this information as a JSON document>>.
+Be aware, that during startup endpoint processing, all the collected steps will be pulled from the buffer, and sent in the response - so the further requests won't get the startup steps again.
 
 [[boot-features-external-config]]
 == Externalized Configuration