Previous versions of Jenkins show a help button for the "Label Expression" field under "Restrict where this job can run" in the job configuration page.

With Jenkins 1.609.1, I no longer see this help button or text on the job configuration page.

The text itself does appear to be present under http://jenkins/descriptor/hudson.model.FreeStyleProject/help/assignedLabelString, but it's not linked on the job configuration page.