Add XML documentation for Android.App.Assist

The `Android.App.Assist` namespace contains **143** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **139** to **143** (increased by **4**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command in docs\xml to list the 143 `To be added` placeholders under Android.App.Assist and inspect the matching source XML documentation files. Replace those placeholders with accurate documentation for the namespace members, then rerun the scan to confirm none remain.

索引モデルが issue の本文から書いたものです。