Improve stat_smooth documentation. (#2681)

Author: clauswilke

R/stat-smooth.r

@@ -4,9 +4,13 @@
 #'   For `method = "auto"` the smoothing method is chosen based on the
 #'   size of the largest group (across all panels). [loess()] is
 #'   used for less than 1,000 observations; otherwise [mgcv::gam()] is
-#'   used with `formula = y ~ mgcv::s(x, bs = "cs")`. Somewhat anecdotally,
+#'   used with `formula = y ~ s(x, bs = "cs")`. Somewhat anecdotally,
 #'   `loess` gives a better appearance, but is O(n^2) in memory, so does
 #'   not work for larger datasets.
+#'
+#'   If you have fewer than 1,000 observations but want to use the same `gam`
+#'   model that `method = "auto"` would use then set
+#'   `method = "gam", formula = y ~ s(x, bs = "cs")`.
 #' @param formula Formula to use in smoothing function, eg. `y ~ x`,
 #'   `y ~ poly(x, 2)`, `y ~ log(x)`
 #' @param se Display confidence interval around smooth? (TRUE by default, see