`xs` (cross-section) operation missing KDocs

[Jolanrensen]

xs (cross-section) operation is still missing KDocs. I keep forgetting what it does XD

The website could also use some improvement. https://kotlin.github.io/dataframe/xs.html

What I think could help most is some rewrites into other functions, to see what xs is shorthand for.
For instance, that

df.xs("a", "b")
// is equivalent to
df.xs("a", "b") { df.flatten().columns().take(2) }

and that

df.xs(1, 2) { colA and colB }
// is equivalent to
df
    .filter { colA == 1 && colB == 2 }
    .remove { colA and colB }