Expand JS documentation.

1. Make it clear that "minimum latency" != "run ASAP after latency"
2. Note that deadline constraints are dropped from rescheduled jobs
3. Clarify that jobFinished does not need to be called if onStopJob is called

Bug: 19536175
Test: Run `m offline-sdk-docs` and check site at out/target/common/docs/offline-sdk/reference/android/app/job/JobInfo.html
Change-Id: I051feb0ebfcb0a24c452a38b91a751bbeef581f8