add tests and update documentation

Author: behramulukir

NEWS.md

@@ -1,5 +1,7 @@
 # bayesplot (development version)
 
+* Add `shape` argument to `mcmc_scatter()` by @behramulukir (#375)
+
 * PPC "avg" functions (`ppc_scatter_avg()`, `ppc_error_scatter_avg()`, etc.) gain a `stat` argument to set the averaging function. (Suggestion of #348, @kruschke).
 * `ppc_error_scatter_avg_vs_x(x = some_expression)` labels the *x* axis with `some_expression`.
 * Add `ppc_dots()` and `ppd_dots()` by @behramulukir (#357)

R/mcmc-scatterplots.R

@@ -11,7 +11,7 @@
 #' @template args-regex_pars
 #' @template args-transformations
 #' @param ... Currently ignored.
-#' @param size,alpha For `mcmc_scatter()`, passed to
+#' @param shape,size,alpha For `mcmc_scatter()`, passed to
 #'   [ggplot2::geom_point()] to control the appearance of the points.
 #' @param bins,binwidth For `mcmc_hex()`, an optional numeric vector of
 #'   *length two* passed to [ggplot2::geom_hex()] to override the