cmd/gofmt: document -s transformations

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/13721043

src/cmd/gofmt/doc.go

@@ -71,6 +71,25 @@ To remove the parentheses:
 To convert the package tree from explicit slice upper bounds to implicit ones:
 
 	gofmt -r 'α[β:len(α)] -> α[β:]' -w $GOROOT/src/pkg
+
+The simplify command
+
+When invoked with -s gofmt will make the following source transformations where possible.
+
+	An array, slice, or map composite literal of the form:
+		[]T{T{}, T{}}
+	will be simplified to:
+		[]T{{}, {}}
+
+	A slice expression of the form:
+		s[a:len(s)]
+	will be simplified to:
+		s[a:]
+
+	A range of the form:
+		for x, _ = range v {...}
+	will be simplified to:
+		for x = range v {...}
 */
 package main