mention what happens with non-existent elements

git-svn-id: https://svn.r-project.org/R/trunk@87454 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

src/library/base/man/Extract.Rd

@@ -1,6 +1,6 @@
 % File src/library/base/man/Extract.Rd
 % Part of the R package, https://www.R-project.org
-% Copyright 1995-2022 R Core Team
+% Copyright 1995-2024 R Core Team
 % Copyright 2002-2022 The R Foundation
 % Distributed under GPL 2 or later
 
@@ -204,6 +204,8 @@ 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}.
+
   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)}.