Doc fixes for pairs in Select/MultiSelect

[yha]

Clarify the docs and correct the types the Select and MultiSelect (I suspect there was some confusion in https://github.com/JuliaPluto/PlutoUI.jl/commit/18786e10f95e28a8f8cc299996291ff2a417d421 where ::String was moved from key to value rather than being removed)

[github-actions[bot]]

Try this Pull Request!

Open Julia and type:

  julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/yha/PlutoUI.jl", rev="yha/select-docs")
julia> using PlutoUI

Or run this code in your browser:

[fonsp]

Thanks!