When editing a custom thread field, the description allows for multiple lines and formatting with CR/LF
But when creating a thread, the custom thread field's description is 'trimmed' and doesn't follow the CR/LF formatting when editing it.
You can use HTML here, so you'd insert line breaks (<br />) to accomplish the desired effect. It might be worth adding a hint to this field to make this clearer, however.
Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XF release (2.2.0 Beta 6).
Change log:
Add a hint to the custom field description input to clarify HTML is allowed