[FR] create syntax myplot!(p, args...; kwargs...) for user recipes

[matthieugomez]

The docs for RecipeBases suggests to define a user recipe as follows:

mutable struct MyPlot
    args
end
export myplot, myplot!
myplot(args...; kw...) = plot(MyPlot(args); kw...)
myplot!(args...; kw...) = plot!(MyPlot(args); kw...)

To be more consistent with the ecosystem, I think the docs (and the macro @userplot) should add the following definition:

myplot!(p::AbstractPlot, args...; kw...) = plot!(p, MyPlot(args); kw...)

[matthieugomez]

the macro already does it actually. opened a PR in plotdocs to correct the docs.
JuliaPlots/PlotDocs.jl#355