Documentation on default arguments

From slack. Point out that an rrule is basically a new dispatch. So if you have any default arguments, you will need to redefine them. For example

using Zygote, ChainRulesCore
function f(x, y=0.0; a=2)
    return a * x + y
end
function ChainRulesCore.rrule(::typeof(f), x, y=0.0; a=2)
    r = f(x, y; a)
    function f_pb(Δr)
        # implementation, which can use the `y`, and `a`
    end
    return r, f_pb
end

Can point out that the kwargs on the dispatch is useful to slurp up keyword arguments to pass to the f(...) in the primal but you would need to separate out any values with defaults, as you would implementing any other dispatch.

Start by locating the documentation covering rrule dispatches and default arguments. Explain that an rrule is a new dispatch, so default positional and keyword arguments must be redefined, while kwargs can collect keyword arguments for the primal call. Done means the documentation includes the Julia example and clearly describes separating defaulted values.