Added documentation for parameters used by CanBusAnalogServer

I had to annotate the meaning of the CanBusAnalogServer for personal use, so I thought it could make sense to have them in the official documentation. 

@randaz81 If you have time,  can you do a brief check that everything is correct?

@lornat75 Currently we miss a convention on how to document parameters in yarp devices, using the markdown table syntax a nice and readable table is generated:
![parameter_doc](https://cloud.githubusercontent.com/assets/1857049/4370441/ee9f12ec-4308-11e4-844d-c52c06eb16d8.png)

done.