Update setNotification documentation.

Clarify what the contents of the notification should be and fix an API
link.

Bug: 19536175
Test: Run `m offline-sdk-docs` and check page at out/target/common/docs/offline-sdk/reference/android/app/job/JobService.html
Change-Id: Ic1471eb28097aff99567b3b9425f173da569f614