context: Be clear that users must cancel the result of WithCancel.

This is necessary for cleanup of any goroutine that blocks on
ctx.Done(), as is done in e.g. propagateCancel in context.go. It is
already specified for WithTimeout and WithDeadline, whose descriptions I
have simplified here.

Change-Id: I6b30605decb8c0b7c1296c3f8852f9ad305ad4b7
Reviewed-on: https://go-review.googlesource.com/5022Reviewed-by: Sameer Ajmani <sameer@golang.org>

context/context.go

@@ -205,6 +205,9 @@ type CancelFunc func()
 // WithCancel returns a copy of parent with a new Done channel. The returned
 // context's Done channel is closed when the returned cancel function is called
 // or when the parent context's Done channel is closed, whichever happens first.
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete.
 func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
 	c := newCancelCtx(parent)
 	propagateCancel(parent, &c)
@@ -343,9 +346,8 @@ func (c *cancelCtx) cancel(removeFromParent bool, err error) {
 // cancel function is called, or when the parent context's Done channel is
 // closed, whichever happens first.
 //
-// Canceling this context releases resources associated with the deadline
-// timer, so code should call cancel as soon as the operations running in this
-// Context complete.
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete.
 func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) {
 	if cur, ok := parent.Deadline(); ok && cur.Before(deadline) {
 		// The current deadline is already sooner than the new one.
@@ -405,9 +407,8 @@ func (c *timerCtx) cancel(removeFromParent bool, err error) {
 
 // WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
 //
-// Canceling this context releases resources associated with the deadline
-// timer, so code should call cancel as soon as the operations running in this
-// Context complete:
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete:
 //
 // 	func slowOperationWithTimeout(ctx context.Context) (Result, error) {
 // 		ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)