Add tip for form_header attribute

[Bubballoo3]

Idea from this discourse thread https://discourse.openondemand.org/t/hovering-issue-of-manifest/4679, since it seems like people may not be able to find the form_header attribute if they expect to find it in the manifest page (since it is an alternative to description in many cases). As such we should add a tip below the description explicitly stating that 'If you want a description that only renders on the form and not on popups, see the form_header attribute'