net/context: Don't leak goroutines in Done example.

The current example leaks the DoSomethingSlow goroutine for an
arbitrarily long time.  In a real server, that can lead to an
out-of-memory failure during events such as network outages; a
more careful version of that example would be too long for a
simple package doc.

Fortunately, there are other short, common patterns using Done
that don't leak and don't require a lot of explanation.  Let's
use one of those instead.

Change-Id: I0ad0c6121d06b757a397e0e71be9e01ccfd75f77
Reviewed-on: https://go-review.googlesource.com/4490Reviewed-by: Andrew Gerrand <adg@golang.org>

context/context.go

@@ -64,16 +64,19 @@ type Context interface {
 	//
 	// Done is provided for use in select statements:
 	//
-	// 	// CancelableOperation calls UncancelableOperation and returns as soon as
-	// 	// it returns or ctx.Done is closed.
-	// 	func CancelableOperation(ctx context.Context) (Result, error) {
-	// 		c := make(chan Result, 1)
-	// 		go func() { c <- UncancelableOperation() }()
+	//  // Stream generates values with DoSomething and sends them to out
+	//  // until DoSomething returns an error or ctx.Done is closed.
+	//  func Stream(ctx context.Context, out <-chan Value) error {
+	//  	for {
+	//  		v, err := DoSomething(ctx)
+	//  		if err != nil {
+	//  			return err
+	//  		}
 	//  		select {
-	// 		case res := <-c:
-	// 			return res, nil
 	//  		case <-ctx.Done():
-	// 			return nil, ctx.Err()
+	//  			return ctx.Err()
+	//  		case out <- v:
+	//  		}
 	//  	}
 	//  }
 	//