[Rd] Extract.data.frame.Rd about $.data.frame

[Martin Maechler]

>>>>> Suharto Anggono Suharto Anggono via R-devel 
>>>>>     on Sun, 10 Feb 2019 16:33:25 +0000 writes:

    > In R devel, data.frame method of '$' has been removed, but this part of "Details" section of Extract.data.frame.Rd still implies existence of the method.
    > The \code{data.frame} method for \code{$}, treats \code{x} as a
    > list, except that (as of R-3.1.0) partial matching of \code{name} to
    > the names of \code{x} will generate a warning; this may become an
    > error in future versions.  The replacement method checks
    > \code{value} for the correct number of rows, and replicates it if
    > necessary.


    > Statement from before R 3.1.0 could be used again:

    > There is no \code{data.frame} method for \code{$}, so \code{x$name}
    > uses the default method which treats \code{x} as a list.  There is a
    > replacement method which checks \code{value} for the correct number
    > of rows, and replicates it if necessary.

Thanks a lot, Suharto, for finding and reporting this!
I've added a 2 x 2 words of explanation to make it easier to understand.

Now changed.
Martin