Add warning about using mmaparrays keyword

[m-wells]

The current behavior is to silently accept the keyword.

import HDF5, JLD, JLD2

function writetest(f::Function, g::Function; kwargs...)
    y = f("foo", "w"; kwargs...) do j
        x = ones(Int,2)
        j["x"] = x
        @show x
        x
    end

    f("foo", "r+"; kwargs...) do j
        x = g(j["x"])
        x[2] = 10 
        @show x[:]
    end

    f("foo", "r"; kwargs...) do j
        x = g(j["x"])
        @show x[:]
    
       if y == x
           println("did NOT update")
       else
           println("did update")
       end
   end
end

function mmaptest()
    println("HDF5")
    writetest(HDF5.h5open, HDF5.readmmap)
    println("JLD")
    writetest(JLD.jldopen, x -> x; mmaparrays=true)
    println("JLD2")
    writetest(JLD2.jldopen, x -> x; mmaparrays=true)
end

And the result

julia> mmaptest()
HDF5
x = [1, 1]
x[:] = [1, 10]
x[:] = [1, 10]
did update
JLD
x = [1, 1]
x[:] = [1, 10]
x[:] = [1, 10]
did update
JLD2
x = [1, 1]
x[:] = [1, 10]
x[:] = [1, 1]
did NOT update

[JonasIsensee]

This seems quite reasonable.
Thank you!