Some users don't realize that creating a Context with a CancelFunc
attaches a subtree to the parent, and that that subtree is not released
until the CancelFunc is called or the parent is canceled.  Make this
clear early in the package docs, so that people learning about this
package have the right conceptual model.

Change-Id: I7c77a546c19c3751dd1f3a5bc827ad106dd1afbf
Reviewed-on: https://go-review.googlesource.com/24090Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>