tweaks

git-svn-id: https://svn.r-project.org/R/trunk@87458 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

src/library/base/man/Extract.Rd

@@ -204,15 +204,15 @@ x$name <- value
   equivalent to \code{alist[[i1]]\dots[[ip]]} providing all but the
   final indexing results in a list.
 
-  Attempts to extract a non-existent element return \code{NULL}.
+  Attempts to extract a non-existent element by name return \code{NULL}.
 
   Note that in all three kinds of replacement, a value of \code{NULL}
   deletes the corresponding item of the list.  To set entries to
   \code{NULL}, you need \code{x[i] <- list(NULL)}.
 
   When \code{$<-} is applied to a \code{NULL} \code{x}, it first coerces
   \code{x} to \code{list()}.  This is what also happens with \code{[[<-}
-  where in \R versions less than 4.y.z, a length one value resulted in a
+  where in \R versions before 4.0.0, a length one value resulted in a
   length one (atomic) \emph{vector}.
 }