Update documentation for iaf_psc_exp

[ackurth]

No description provided.

[heplesser]

@ackurth This looks quite good already, but see my inline comments. I would also like to ask for a slight re-ordering of material that you have not touched:

1. Move the last paragraph before the Parameters section ("For conversion ...") before the note on the time constants.
2. Put the material on the two different ways in which the model can handle input into a note as well.

[jessica-mitchell]

Here is link to output on Read the Docs: https://nest-simulator--3084.org.readthedocs.build/en/3084/models/iaf_psc_exp.html

[jessica-mitchell]

@ackurth @heplesser After some discussion with David D. about models, we noticed that in the original model description, it mentions the stochastic use case - which does not appear to be in this version - is that correct?

Spiking in this model can be either deterministic (delta=0) or stochastic (delta > 0). In the stochastic case this model implements a type of spike response model with escape noise

[jessica-mitchell]

thanks @ackurth, just a couple of minor typos

[heplesser]

@ackurth Thanks, this is fine now except for the small details @jessica-mitchell has pointed out. Please merge master so that tests will pass.