Document mapping arg to geom_abline() without mentioning inherit.aes (#3173)

Author: karawoo

R/geom-abline.r

@@ -33,6 +33,7 @@ NULL
 #'   adding straight line segments to a plot.
 #' @inheritParams layer
 #' @inheritParams geom_point
+#' @param mapping Set of aesthetic mappings created by [aes()] or [aes_()].
 #' @param xintercept,yintercept,slope,intercept Parameters that control the
 #'   position of the line. If these are set, `data`, `mapping` and
 #'   `show.legend` are overridden.