Chris, if you take a look at the default templates, you should find a good modal window example. It allows you to create an editable text are on the form along with a button on the form that pops up the instructions over / above the form.
Alternatively, if you need author-editable text inside your form, then you can add a parsys section to the template of your form. That will allow you to have an author editable area on the form that you can either use the editor for, or will be stored as a crx node that you can edit outside of the form editor (<cq:include path="par" resourceType="foundation/components/parsys" />).