Formatting specific text within a standard multiline textbox is not really possible; this is not a limitation of Forms, it is just the nature of the HTML textarea element.

You can use CSS to bold or underline the entire contents of the field, but not specific pieces. You could probably format it a bit more on the saved copy because it gets converted to text, but that still doesn't allow the submitter to add any kind of format/styling.

However, Forms 10.4 added a Rich Text field type, which does support this functionality (and more).