gofmt/doc: gofmt -s output may not be backward compatible

Change-Id: If697ab554e6cb5545d99c6b103ed8bc54f69ed48
Reviewed-on: https://go-review.googlesource.com/4161Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/gofmt/doc.go

@@ -87,6 +87,13 @@ When invoked with -s gofmt will make the following source transformations where
 		for x, _ = range v {...}
 	will be simplified to:
 		for x = range v {...}
+
+	A range of the form:
+		for _ = range v {...}
+	will be simplified to:
+		for range v {...}
+
+This may result in changes that are incompatible with earlier versions of Go.
 */
 package main