DimStack `keyword arguments` are undocumented

[alex-s-gardner]

There is no documentation for DimStack keyword arguments

https://rafaqz.github.io/DimensionalData.jl/stable/api/reference#DimensionalData.DimStack

[tiemvanderdeure]

Now that we're at it, the whole docstring needs an update, because the maplayers updates aren't reflected in the docstring at all:

• iteration and map apply over array layers, as indexed with a Symbol. Is not true anymore
• first(dimstack) will return the first value of the iterator - the DimArray for the first layer is also not true anymore
• other places we just need to replace map by maplayers (which isn't mentioned at all and probably should)

[rafaqz]

Yeah, needs a serious overhaul