You should document your functions using a string at the beginning. This is called docstring:

def newton(f, x0):
    """
    Newton's method for computing a zero of a function
    on input:
    f  (function) given function f(x)
    x0 (float) initial guess 
    on return:
    y  (float) the approximated zero of f
    """
     ...

When calling help(newton), you get this docstring displayed together with the call of this function:

Help on function newton in module __main__:

newton(f, x0)
     Newton's method for computing a zero of a function
     on input:
     f  (function) given function f(x)
     x0 (float) initial guess
     on return:
     y  (float) the approximated zero of f

The docstring is internally saved as an attribute, __doc__, of the given function. In the example, it's newton.__doc__. The minimal information you should provide in a docstring is the purpose of the function...