time: clarify why timer.Stop and ticker.Stop don't close the channel

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6818106

src/pkg/time/sleep.go

@@ -36,6 +36,8 @@ type Timer struct {
 // Stop prevents the Timer from firing.
 // It returns true if the call stops the timer, false if the timer has already
 // expired or stopped.
+// Stop does not close the channel, to prevent a read from the channel succeeding
+// incorrectly.
 func (t *Timer) Stop() (ok bool) {
 	return stopTimer(&t.r)
 }

src/pkg/time/tick.go

@@ -39,6 +39,8 @@ func NewTicker(d Duration) *Ticker {
 }
 
 // Stop turns off a ticker.  After Stop, no more ticks will be sent.
+// Stop does not close the channel, to prevent a read from the channel succeeding
+// incorrectly.
 func (t *Ticker) Stop() {
 	stopTimer(&t.r)
 }