time: clarify documentation for Location.String

Change-Id: I5c32a968303848304cbd6fe1e83bed0af11f6440
Reviewed-on: https://go-review.googlesource.com/48960Reviewed-by: Caleb Spare <cespare@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/zoneinfo.go

@@ -82,7 +82,7 @@ func (l *Location) get() *Location {
 }
 
 // String returns a descriptive name for the time zone information,
-// corresponding to the argument to LoadLocation.
+// corresponding to the name argument to LoadLocation or FixedZone.
 func (l *Location) String() string {
 	return l.get().name
 }