Clarify that span is only used for loess in stat_smooth (#4261)

* Clarify that span is only used for loess in stat_smooth

It can be surprising that `span` argument has no effect when number of observations is large.

* Change docstring in .r file

* Apply suggestions from code review

Co-authored-by: Hiroaki Yutani <yutani.ini@gmail.com>

* Document

Co-authored-by: Hiroaki Yutani <yutani.ini@gmail.com>
Co-authored-by: GitHub Actions <actions@github.com>

Author: 3 people

R/stat-smooth.r

@@ -25,7 +25,9 @@
 #' @param level Level of confidence interval to use (0.95 by default).
 #' @param span Controls the amount of smoothing for the default loess smoother.
 #'   Smaller numbers produce wigglier lines, larger numbers produce smoother
-#'   lines.
+#'   lines. Only used with loess, i.e. when `method = "loess"`,
+#'   or when `method = NULL` (the default) and there are fewer than 1,000
+#'   observations.
 #' @param n Number of points at which to evaluate smoother.
 #' @param method.args List of additional arguments passed on to the modelling
 #'   function defined by `method`.