describe dif strategies used by position_dodge and position_dodge2 (tidyverse#3784)

* attempted to clarify language here

* updated documentation

Author: Danielle Quinn

R/position-dodge.r

@@ -1,10 +1,12 @@
 #' Dodge overlapping objects side-to-side
 #'
 #' 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. But unlike `position_dodge`,
+#' horizontal position. `position_dodge` requires the grouping variable to be
+#' be specified in the global or `geom_*` layer. Unlike `position_dodge`,
 #' `position_dodge2` works without a grouping variable in a layer.
+#' `position_dodge2` works with bars and rectangles, but is
+#' particulary useful for arranging box plots, which
+#' can have variable widths.
 #'
 #' @param width Dodging width, when different to the width of the individual
 #'   elements. This is useful when you want to align narrow geoms with wider