clarify some ambiguity in `in` docstring (#61892)

otherwise it might be quite confusing that e.g. 
```julia
julia> (:a => NaN) in Dict(:a => NaN)
false

julia> (:a => missing) in Dict(:a => missing)
missing
```

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Co-authored-by: Shuhei Kadowaki <40514306+aviatesk@users.noreply.github.com>