Document kwargs and fix missing uses

The kwargs `absstep`, `relstep` and `dir` are not uniformly documented. And there might even be cases where one kwarg is forgotten, i.e. [here](https://github.com/JuliaDiff/FiniteDiff.jl/blob/a7eca2d4b73c4de12140d89df7621fcc90d29190/src/gradients.jl#L183-L194) (`dir` is missing). Now that DI has to use them, it would be nice if someone could explain what they do and check that they are properly passed around?