fix documentation for position_dodge and position scales (#3418, fixes 2767)

* added link to sec_axis() in sec.axis parameter documentation and removed it from see also section

* added documentation on position_dodge2

Author: CateGitau

R/position-dodge.r

@@ -3,7 +3,8 @@
 #' Dodging preserves the vertical position of an geom while adjusting the
 #' horizontal position. `position_dodge2` is a special case of `position_dodge`
 #' for arranging box plots, which can have variable widths. `position_dodge2`
-#' also works with bars and rectangles.
+#' also works with bars and rectangles. But unlike `position_dodge`,
+#' `position_dodge2` works without a grouping variable in a layer.
 #'
 #' @inheritParams position_identity
 #' @param width Dodging width, when different to the width of the individual

R/scale-continuous.r

@@ -70,9 +70,8 @@ NULL
 
 #' @rdname scale_continuous
 #'
-#' @param sec.axis specify a secondary axis
+#' @param sec.axis [sec_axis()] is used to specify a secondary axis.
 #'
-#' @seealso [sec_axis()] for how to specify secondary axes
 #' @export
 scale_x_continuous <- function(name = waiver(), breaks = waiver(),
                                minor_breaks = waiver(), labels = waiver(),