Mention in doc that new_rcrd() requires a format method (#1954)

R/type-rcrd.R

@@ -8,6 +8,12 @@
 #' length, like [POSIXlt], but where the organisation should be considered
 #' an implementation detail invisible to the user (unlike a [data.frame]).
 #'
+#' @details
+#' Record-style objects created with [new_rcrd()] do not do much on their own.
+#' For instance they do not have a default [format()] method, which means printing
+#' the object causes an error. See [Record-style objects](https://vctrs.r-lib.org/articles/s3-vector.html?q=record#record-style-objects
+#' for details on implementing methods for record vectors.
+#'
 #' @param fields A list or a data frame. Lists must be rectangular
 #'   (same sizes), and contain uniquely named vectors (at least
 #'   one). `fields` is validated with [df_list()] to ensure uniquely