Add XML documentation for Android.App.Job

The `Android.App.Job` namespace contains **387** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

```powershell
rg -n -i --glob 'Android.App.Job/**' --glob 'ns-Android.App.Job.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **377** to **387** (increased by **10**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the provided rg command to enumerate the 387 placeholders under docs\xml for Android.App.Job, excluding docs\xml\index.xml. Read the matching Android.App.Job and ns-Android.App.Job.xml files to understand the existing XML documentation style. Done means the placeholders in this namespace have been replaced with documentation and the scan no longer reports them.

Written by the indexing model from the issue text.