Merge pull request #4067 from jsoref/JENKINS-57992

[JENKINS-57992] clarify that the system user is a jenkins concept

core/src/main/resources/jenkins/security/QueueItemAuthenticatorMonitor/message.properties

-blurb = Builds in Jenkins run as the virtual SYSTEM user with full permissions by default. \
+blurb = Builds in Jenkins run as the virtual Jenkins SYSTEM user with full Jenkins permissions by default. \
   This can be a problem if some users have restricted or no access to some jobs, but can configure others. \
   If that is the case, it is recommended to install a plugin implementing build authentication, and to override this default.