The easiest thing to do is direct your volunteers to a survey page after finishing the volunteer signup. Directing automatically to a new page upon completion of the first form is an option under “Volunteer settings.” 

With the survey page type, you can create the kind of short answers you’d like to collect, and it’s easy to review and even export them. Be sure to set correct privacy settings when creating the survey, based on whether or not you want participants to be able to publicize their answers