Enhance and clean up model documentation

[jougs]

This PR addresses the remaining open action items and thereby fixes #1620.

[jougs]

@terhorstd, @jessica-mitchell: friendly ping!

[jessica-mitchell]

@jougs looks good, I just made some suggestions to mainly adding double back ticks in a few places

[jougs]

@jessica-mitchell: I have applied most of your suggestions and have two questions for the as-of-yet unapplied ones (mostly regarding SetDefaults):

• Instead of just making them typewriter by double-backticking them, what about marking the functions with :py:func:, so they are auto-linked to the PyNEST API?
• Is there a similar mechanism (existing/planned) for the models? The links into the model directory are also pretty much generic after all.

[jessica-mitchell]

* Instead of just making them typewriter by double-backticking them, what about marking the functions with `:py:func:`, so they are auto-linked to the PyNEST API?

Ah yes, that would make more sense, I just wasn't thinking about it

* Is there a similar mechanism (existing/planned) for the models? The links into the model directory are also pretty much generic after all.

Yes there is a way to do this but not implemented in any way yet;i think this could be something we look at in a future PR though

[jougs]

@jessica-mitchell: I've addressed all remaining issues as you suggested. You get to decide if you actually find the second review necessary for this one, or if your approval would be sufficient to merge.

[jessica-mitchell]

@jougs thanks! looks good to me and considering these are basically formatting issues, I'd say one review is enough