Guidance + tooling around handling functions that dispatch on a particular type

[ashton314]

See feedback from RxInfer.jl#116 and their datavars comment.

During the development of FloatTracker we ran into lots of functions like floatmin, eps, one, etc. (see TrackedFloat.jl for more examples, search "Type-based functions") that we had to wrap specifically to handle the TrackedFloatN types.

We should

1. Improve the docs on handling functions like this in third-party code
2. Maybe provide some tooling to make that work easier/more automatic for users of FloatTracker