time: document that time.Tick creates an unrecoverable resource

Fixes #11662.

Change-Id: I8839b48a1cd3da4c29b6def03cb0c74c1c12942e
Reviewed-on: https://go-review.googlesource.com/12066Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/tick.go

@@ -47,7 +47,9 @@ func (t *Ticker) Stop() {
 }
 
 // Tick is a convenience wrapper for NewTicker providing access to the ticking
-// channel only.  Useful for clients that have no need to shut down the ticker.
+// channel only. While Tick is useful for clients that have no need to shut down
+// the Ticker, be aware that without a way to shut it down the underlying
+// Ticker cannot be recovered by the garbage collector; it "leaks".
 func Tick(d Duration) <-chan Time {
 	if d <= 0 {
 		return nil