Doc correction

[harrahx3]

I found small errors in the doc:

• For the lines the parameter x_ohm_per_km corresponds to the reactance of the line and not its inductance (else the unit would have been h_per_km)
• In the docstring of the create_load and create_loads functions the parameter p_mw was described as optional and defaulting to 0, but it is not (but maybe it should ?)

Also there is a contradiction between the comment in this docstring that says that a negative load can be used to model a static generator, and the note in the documentation: "Loads should always have a positive p_mw value, since all power values are given in the consumer system. If you want to model constant generation, use a Static Generator (sgen element) instead of a negative load.".

Is using a load with a negative p_mw equivalent to using a sgen element or is there any difference between the two (apart from the power signing convention) ?