Update stdout and stderr argument docs (#283)

R/eval.R

@@ -44,17 +44,21 @@
 #'   then the commands are not echoed and will not be shown
 #'   in the standard output. Also note that you need to call `print()`
 #'   explicitly to show the output of the command(s).
-#'   IF `NULL` (the default), then standard output is not returned, but
+#'   IF `NULL`, then standard output is not returned, but
 #'   it is recorded and included in the error object if an error happens.
+#'   Various special values for this argument such as `"|"` are explained
+#'   in the `stdout` argument of [processx::process].
 #' @param stderr The name of the file the standard error of
 #'   the child R process will be written to.
 #'   In particular `message()` sends output to the standard
 #'   error. If nothing was sent to the standard error, then this file
 #'   will be empty. This argument can be the same file as `stdout`,
 #'   in which case they will be correctly interleaved. If this is the
 #'   string `"2>&1"`, then standard error is redirected to standard output.
-#'   IF `NULL` (the default), then standard output is not returned, but
+#'   IF `NULL`, then standard output is not returned, but
 #'   it is recorded and included in the error object if an error happens.
+#'   Various special values for this argument such as `"|"` are explained
+#'   in the `stdout` argument of [processx::process].
 #' @param error What to do if the remote process throws an error.
 #'   See details below.
 #' @param poll_connection Whether to have a control connection to