exp/locale/collate: clarification in comments on use of returned value.

R=r
CC=golang-dev
https://golang.org/cl/6752043

src/pkg/exp/locale/collate/collate.go

@@ -86,6 +86,7 @@ type Collator struct {
 }
 
 // Locales returns the list of locales for which collating differs from its parent locale.
+// The returned value should not be modified.
 func Locales() []string {
 	return availableLocales
 }