Documentation for `NA` term

[koperagen]

This term is used in valueCounts and statistical operations: std, mean, varianceAndMean
https://kotlin.github.io/dataframe/valuecounts.html
In documentation, there is no explanation of what it means. What's the difference from NaN?
In the following function, skipNA used when the value is NaN.

public fun Sequence<Float>.mean(skipNA: Boolean = skipNA_default): Double {
    var count = 0
    var sum: Double = 0.toDouble()
    for (element in this) {
        if (element.isNaN()) {
            if (skipNA) continue
            else return Double.NaN
        }
        sum += element
        count++
    }
    return if (count > 0) sum / count else Double.NaN
}

[Jolanrensen]

I mentioned it in the KDocs in #214, but indeed, we need it on the documentation website too.