Improve docstrings for phase differences calculations

[Jellevanderwerff]

Both for thebeat.stats.get_phase_differences and thebeat.visualization.plot_phase_differences the docstring is lacking. The window_size argument is for instance completely undescribed.

[YannickJadoul]

Is this already resolved in 8551b2e?

[Jellevanderwerff]

Nope! That was just the list of parameters, will have to take some time for a proper theoretical explanation