Improve usage description for BufferedApplicationStartup

[ascheman]

I was trying the nice new Spring Boot startup tracking feature and used BufferedApplicationStartup to collect initialization data. I found that the collected data is drained from the buffer after each request. In particular, it took me a moment to recognize that the most important data is lost after the first request. As discussed with @bclozel this could be emphasized in the documentation, cf. https://stackoverflow.com/questions/65413579/spring-boot-bufferedapplicationstartup-drained-after-first-request

[oleg-sukhov]

Hello, @snicoll. I would like to make my first contribution to spring boot project. Do you think that this issue is a good start point? If yes, what should I do to start working on it?

[snicoll]

@oleg-sukhov thanks for asking. If you've used the startup feature and see what needs to be documented, please go ahead and submit a PR. The documentation is located in spring-boot-project/spring-boot-docs/src/docs/asciidoc in this repository and the contributing page should provide you more details to get started.

[mbhave]

Closing in favor of PR #25075.