Update JS docs.

1. Note some of the EJ changes
2. Clarify behavior of network byte value usage
3. Attempt to reduce confusion of job timeout wording
4. Note priority behavior of important while foreground
5. Clarify prefetch constraint change

Bug: 19536175
Bug: 142272435
Bug: 188429037
Bug: 223437753
Test: Run `m offline-sdk-docs` and check page at out/target/common/docs/offline-sdk/reference/android/app/job/<X>.html
Change-Id: I250fbd40d538abb7a8868b607319093f551b61a9