Add javadoc to CachingUserDetailsService

[FdHerrera]

Add javadoc to CachingUserDetailsService, closes gh-10914

[sjohnr]

@FdHerrera thanks for the PR! I want to mention that based on reviewing gh-10914, I believe we should be updating/enhancing the reference documentation to include information about CachingUserDetailsService. Adding javadoc is good too, though so thanks for doing that. Any chance you could add something to the reference docs? Let me know if you have any questions about it.

[FdHerrera]

I'll figure out how to add a docs, never done that before.
Maybe we can close this MR and add the doc in another one?

[sjohnr]

You can add them under this PR and we can update the title. The docs are located under docs/modules/ROOT/pages and you can preview them by running ./gradlew antora. It will download a Node.js bundle and build the docs into a local docs site under docs/build/site.

[sjohnr]

You can also run ./gradlew format to fix the formatting violation in the javadoc you added.

[FdHerrera]

Sorry, will create a new MR because wasn't able to fix a problem with conflicts in my fork 😅