Your code is your documentation. C'mon we are not writing assembly language!
I'll give you an example. Instead of writing:
interest = principal * .07 * numbeOfYears; // then here you need to explain what's that magic number .07 by writing silly comments
write this instead.
interest = principal * numberOfyears * ANNUAL_INTEREST_RATE;
It's so obvious you dont need a comment.
PS. Writing an API is a different story. You need to put a document what does an API doing. At least the input/parameter, output expected, and the execeptions.