Add more documentation

[hersle]

It's not a lot, but it's something.
Appreciate it if you read through and suggest any changes!
Some of it is intended to close #1822.

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

[ChrisRackauckas]

Looks all good to me. I think we need to add these docstrings to the docs as well, but that doesn't necessarily need to be in this one.

One of the big issues with the ModelingToolkit docs is that you used to only be allowed to use a docstring in one place. So generate_jacobian could only be on one page. But now there's a primary location feature in documenter. So we could have the primary location be some abstractsystem page, but then also have the ODESystem be complete with all of the functions that work on an ODESystem.