Update max priority docs.

Update the docs to note that user-initiated jobs have MAX priority and
give additional examples of max priority tasks.

Bug: 19536175
Test: `m offline-sdk-docs` and view page at out/target/common/docs/offline-sdk/reference/android/app/job/JobInfo.html
Change-Id: I319d03f279f1a435860ff09a0e58e163a1b516a5