[FIXED JENKINS-6153] Add description for label atoms

The change to descriptionForm.jelly is necessary because JEXL
interprets the previous version as a call to the static method
get(String) of LabelAtom if ${it} is of that type.
[JENKINS-6153] Only show configure link for label atoms
[JENKINS-6153] Move @since comment to getter
Merge pull request #1380 from daniel-beck/JENKINS-6153

[FIXED JENKINS-6153] Add description for label atoms