Update doc in ppc-discrete.R

[ci skip]

Author: jgabry

R/ppc-discrete.R

@@ -30,9 +30,10 @@
 #'
 #' @template return-ggplot
 #'
-#' @details For all of these plots \code{y} and \code{yrep} must be non-negative
-#'   integers, although they need not be integers in the strict sense of \R's
-#'   \code{\link{integer}} type.
+#' @details For all of these plots \code{y} and \code{yrep} must be 
+#'   integers, although they need not be integers in the strict sense 
+#'   of \R's \code{\link{integer}} type. For rootogram plots \code{y} 
+#'   and \code{yrep} must also be non-negative.
 #'
 #' @section Plot Descriptions:
 #' \describe{