Document iterator.iter field

[gwater]

based on https://discourse.julialang.org/t/callback-at-every-3-integrator-steps-in-differential-equations-jl/56775/5

[ChrisRackauckas]

To do this, we need Sundials to get a getproperty overload to make iter appear there properly.

[gwater]

Ah, so maybe in the meantime something like

iter - the number of steps so far (only available for OrdinaryDiffEq solvers)

[ChrisRackauckas]

No, if we commit it to the interface, then it should be made uniform throughout the interface.