doc: add Jakes, COST 231 and Random propagation model documentations

This is a proposition for completing the propagation models documentation for both Jakes and COST231, which were empty and marked as "To do".

The model description of Jakes and COST231 were based on their doxygen documentation, the code implementation and literature provided with the description of the class (which was added to the references).

As for other models that could benefit a bit of added context/informations:

• The RandomPropagationLossModel takes from the ns3 RNG, which returns a double. This means that essentially, since the possible values are likely far from zero, it is a link-up or link-down randomness rather than a value of loss (between 0 and -100) randomness. Was this the intent of the function ? Since the value may take positive values, this may give very high power at reception. Anyhow, if anybody know the original intent of this function, please tell me how to better describe it.
• The previous point also applies to RandomPropgationDelayModel.

I will make visual for this documentation at a later instance but I wanted to make sure the text was validated first.