[Rd] update R Internals for R 4.0.0

[Jan Gorecki]

Hi R developers,

R ints: https://cran.r-project.org/doc/manuals/r-devel/R-ints.html

At the beginning there is
"this version is for the 3.x.y series."

I would like to kindly ask for update manual for 4.x.y series, which
would also need to cover entries about NAMED, that have been
superseded by REFCNT. There might be of course other places that needs
update, but this one is important feature of R 4.0.0.

Thank you,
Jan Gorecki