Service docs should indicate that a running Task does not prevent it from being GCed

See RT-39493 for the motivation behind this. The test program in RT-39493 (which is not a bug) fails to call a listener on the Service because the Service has gone out of scope and has been garbage collected.

The Service docs should indicate that Task objects do not hold a hard reference to the Service, and that even a running Task will not prevent the Service that started the Task from being garbage collected.