Parameter description is rendered as text instead of HTML.
I screenshot 'aaa' is a string, bbb is a 'validating string'. Both have the same description.
This seems to be the same issue as this https://issues.jenkins-ci.org/browse/JENKINS-45029
Note that this renders as expected on Hudson.
The link 'ERROR' displayed in the image may be related to this: https://issues.jenkins-ci.org/browse/JENKINS-23729 but I did not investigate that yet.