Add XML documentation for Android.Service.Notification

The `Android.Service.Notification` namespace contains **431** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **435** to **431** (reduced by **4**). Related documentation PRs: [#614](https://github.com/dotnet/android-api-docs/pull/614), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command from the repository root to enumerate the 431 placeholders under docs\xml, excluding docs\xml\index.xml. Read the matching Android.Service.Notification XML files and existing nearby entries for context. Done means the scoped “To be added” placeholders have been replaced with XML documentation and the scan confirms none remain.

Written by the indexing model from the issue text.