Add details to start parameter of coord_polar (#3893). (#4039)

Fix #3893

Author: brianrice2

R/coord-polar.r

@@ -4,7 +4,8 @@
 #' are a stacked bar chart in polar coordinates.
 #'
 #' @param theta variable to map angle to (`x` or `y`)
-#' @param start offset of starting point from 12 o'clock in radians
+#' @param start Offset of starting point from 12 o'clock in radians. Offset
+#'   is applied clockwise or anticlockwise depending on value of `direction`.
 #' @param direction 1, clockwise; -1, anticlockwise
 #' @param clip Should drawing be clipped to the extent of the plot panel? A
 #'   setting of `"on"` (the default) means yes, and a setting of `"off"`