This effort is follow-up of JENKINS-26671 and can solve JENKINS-28006 at least partially.

Current behavior is a new description is always concatenated with the previous one. In addition, description from build phase(s) is mixed with the post-build description as well.

I'm going to change that behavior and leave the user to decide when description should be added like a concatenation (current state) and when the description should rewrite it.