plot_ribbon_em() has docs but no @export

[PietrH]

During b-cubed review, from b9e68fd I noticed plot_ribbon_em() has rendered docs, and is available, but does not have an @export roxygen2 tag.

This came up with checkhelper::find_missing_tags()

#' Plot time series with confidence limits and emerging status
#'
#' @param df_plot df. A data.frame containing data to plot.
#' @param x_axis character. Name of column containing x-values. Default:
#'   \code{"year"}.
#' @param y_axis character. Name of column containing y-values. Default:
#'   \code{"number of observations"}.
#' @param x_label character. x-axis label. Default: \code{"x"}.
#' @param y_label character. y-axis label. Default: \code{"y"}.
#' @param ptitle character. Plot title. Default: \code{NULL}.
#' @param verbose logical. If \code{TRUE}, informations about possible issues
#'   are returned. Default: \code{FALSE}.
#' @return a ggplot2 plot object.
#' @importFrom dplyr .data %>%
plot_ribbon_em <- function(df_plot,
                           x_axis = "year",
                           y_axis = "obs",
                           x_label = "x",
                           y_label = "y",
                           ptitle = NULL,
                           verbose = FALSE) {