Documentation should describe how OpenSCAD special variables like $fn are handled

[etjones]

As mentioned in Issue #104, SolidPython can handle some of (all of?) OpenSCAD's special variables. But it's not clear in the documentation just how that works. Add this on.

Without looking at the code, the one thing I know is that any OpenSCAD function that takes a $fn argument can take a SolidPython segments argument. Need to look up the other variables and document them all in a clear place

[ali1234]

Per #107, sometimes you have to use slices, not segments.