Add documentation on which variables I can subscribe to

[behrisch]

Reading the main traci docs it is hard to find out hat the variables are and which are subscribable. Ideas:

• Add the symbolic variable names to the docs
• Mention explicitly where parameters are needed
• Explain which work in subscriptions also referring to libsumo

[behrisch]

The idea is that you should be able to subscribe to all getters but at least this should be tested and documented