You can edit your Products & Services in Settings. From there just create your description and add unit pricing if you want, easy if you work in linear feet. That will be available from the dropdown list every time you create a quote. That is all doable on the mobile app as well, my workflow just doesn't flow that way at the moment.
You can also create a new product called "Terms and Conditions" for example, add whatever text you want and leave the price blank. That's what I do to specify our lawn service schedule.
More info here: https://help.getjobber.com/hc/en-us/articles/115009735848-Products-Services-List