[Rd] document base::`[[.data.frame`

[Jan Gorecki]

Dear R-devel,

Looking at source of base::`[[.data.frame` we can see a mysterious
handling of subsetting rows and columns at the same time. That does
not seems to be dcoumented anywhere in `[[`. Could we extend
documentation for such data.frame use cases? Or if it is meant to be
used only internally, then maybe better to put that into own internal
function?

Best regards,
Jan Gorecki