I figured out a work-around. I created a PNG of the legend and pasted it into the editor of the Info widget. So now it shows the sub-text beneath the legend, and then some other things. Not the greatest, but it works.
That's a great workaround for some scenarios. Our projects would still benefit to having the ability to add text to the top or bottom a legend in AGOL as we're not using Web App Builder. Not sure if there are any other solutions.