[JENKINS-58743] Document new system properties for DefaultConfidentialStore

[Vlatombe]

User-facing documentation for jenkinsci/jenkins#10235

[gounthar]

Thanks, Vincent! 👍

[jglick]

FWIW this system seems really wrong. If we want a complete, accurate list of system properties (encompassing both core and plugins), we should instead define them as constants with a special annotation, and then use an annotation processor to summarize defined properties into a static file in the JAR, then load these files from the current enabled versions of core & plugins and display them in a live page under Manage Jenkins. (You could also record actual calls to SystemProperties though this would then fail to display properties which for whatever reason had not yet been checked in the current session.) Compare about:config in Firefox or chrome://flags/, etc.