Mention doubly robust ATE in mgrf docstring (#1469)

Author: erikcs

r-package/grf/R/multi_arm_causal_forest.R

@@ -132,7 +132,7 @@
 #' abline(0, -1.5, col = "red")
 #' legend("topleft", c("B - A", "C - A"), col = c("black", "blue"), pch = 19)
 #'
-#' # The average treatment effect of the arms with "A" as baseline.
+#' # A doubly robust estimate (AIPW) of the average treatment effect of the arms.
 #' average_treatment_effect(mc.forest)
 #'
 #' # The conditional response surfaces mu_k(X) for a single outcome can be reconstructed from