You can find this option by going to:
Header & Footer section
Turn on Footer, and add the content
Alternatively, you can set Form Footer Source to Airtable Field and define the field that contains the description under Form Footer Field. This way, each record can show a different description.
The contents of the Form Footer (Field) will show up above the Submit button on your form.
💡Note
The form footer will only appear on the last page when the multi-page mode is turned on.