Add more customization possibilities to the field description

#148 uses `<small>` per the bootstrap docs but that doesn't leave us much customization options.  Only non-block HTML elements can be used.  What if you need more than that?

More discussion: https://level12.slack.com/archives/C02DAPS3T/p1631300245034700

Maybe YAGNI at this point?  Leaving it until we have more concrete use cases which can help us inform the implementation.