Document Empty Recorder Output

[varndorfer]

If no events are recorded for a given element, then no output is produced by the recorders (ex: XMLRecorder::saveSimData()). This can be confusing for new users, for example, when you run a short Neuro simulation, only the endogenously active neurons will output spiking activity. New users may think there is something wrong with their configuration or the simulation because they are missing output for most of the neurons.

We should add documentation to clearly explain that elements will only display in the output if an event is recorded for that element.