Jira allows us to use any valid HTML for custom field description, so we added some simple text and an anchor tag that links to an HTML page containing our help information. We also added a span tag with the proper style class in order to have the nice question mark icon used by the Issue Type and Priority fields.

The data-helplink="local" attribute for the anchor tag ensures that, when the user clicks on the help icon, the help page is opened in a separate page rather than redirecting the current page.