If I write an additional helper method for the jacobi and gauss seidel functions, do I need to add it to the header file and write documentations for it?
I don't think that's necessary. If you want to call the functions from a different code (e.g. if you write your own test program in C), then they have to be in the header file, though.
If I write an additional helper method for the jacobi and gauss seidel functions, do I need to add it to the header file and write documentations for it?