Add XML documentation for Android.App.AppFunctions

The `Android.App.AppFunctions` namespace contains **150** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **146** to **150** (increased by **4**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command against docs\xml to enumerate the 150 `To be added` placeholders under Android.App.AppFunctions, then inspect the matching XML documentation entries and their surrounding API context. Done means replacing all 150 placeholders with useful documentation and confirming the placeholder scan returns no remaining matches.

由索引模型根据 Issue 内容生成。