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/5022
Reviewed-by: Sameer Ajmani <sameer@golang.org>
1 file changed