Formula fields cannot display text formatting, such as bold text.
If you want to have formatting that changes commas to new line characters, you can use the SUBSITUTE function, which is documented in the formula field reference.
If you want a formula that splits items in a list, that is also possible, although a bit of a pain.
If you’d like additional help, please give a bit more information regarding field types (single line text, lookup, etc.) and more detailed example values.