Fix 101 Documentation of current filtering in iaf_psc_exp

[jschuecker]

This PR improves the documentation of the option for synaptic filtering of external current, available in iaf_psc_exp. This addresses #101 .

@heplesser, could you take a look at this?

[heplesser]

@jschuecker Thanks for the more detailed explanation. I would be yet more explicit, e.g.

"iaf_psc_exp can handle current input in two ways: Current input through receptor_type 0 are handled as stepwise constant current input as in other iaf models, i.e., this current directly enters the membrane potential equation. Current input through receptor_type 1, in contrast, is filtered through an exponential kernel with the time constant of the excitatory synapse, tau_syn_ex. For an example application, see [4; reference to paper inserted below]."

I am a bit concerned about the statement that this provides a "direct implementation ... with white noise". Since the noise generator generates stepwise constant input, the input is never really white noise, see doc/model_details/noise_generator.ipynb.

[jschuecker]

@heplesser, I meant "direct" in the sense of a "straight forward" implementation. But you are right, that was not clear and could be misleading. I also included the reference. Thanks!

[heplesser]

👍 and merging.