time: explicitely mention Tickers have to be stopped

LGTM=rsc
R=golang-codereviews, gobot, rsc
CC=golang-codereviews
https://golang.org/cl/64770043

src/pkg/time/tick.go

@@ -17,6 +17,7 @@ type Ticker struct {
 // time with a period specified by the duration argument.
 // It adjusts the intervals or drops ticks to make up for slow receivers.
 // The duration d must be greater than zero; if not, NewTicker will panic.
+// Stop the ticker to release associated resources.
 func NewTicker(d Duration) *Ticker {
 	if d <= 0 {
 		panic(errors.New("non-positive interval for NewTicker"))