Document the precision and limits of sample_laplace_distribution()

The function documentation of sample_laplace_distribution() should answer the following questions:

• what is the range of outputs?
• what is the precision of the outputs?

add_laplace_noise() should document, and possible enforce:

• what is the maximum delta_f / epsilon value that can be used within the limits of double precision arithmetic?
  • to preserve differential privacy, the low bits have to be obscured by the noise. So this can be at most 2^53^.
• what is the minimum delta_f / epsilon value that can be used within the limits of double precision arithmetic?
  • (zero provides no differential privacy)