The description for parameter types such as boolean and string won't save when edited. Editing of the description for an extended choice parameter works fine.
This behaviour works in v1.523 so it's a recent problem, possibly related to JENKINS-22628 Change comments/description to use String.format() instead

I'm wondering if this is also the root cause for JENKINS-23026.

For me this is blocking the roll-out of this version to resolve an issue with AD authentication so although its minor, its really getting in the way.

The work-around is to edit descriptions in the job config.xml and reload the Jenkins configuration from disk.