Add XML documentation for Android.App.AppSearch.Observers

The `Android.App.AppSearch.Observers` namespace contains **72** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **70** to **72** (increased by **2**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

The namespace files under docs\xml\Android.App.AppSearch.Observers/** and ns-Android.App.AppSearch.Observers.xml are the entry points; run the issue's rg scan first to enumerate the 72 placeholders. Read the surrounding API declarations and replace each `To be added` entry with appropriate XML documentation, then rerun the scan and confirm no placeholders remain, excluding docs\xml\index.xml.

由索引模型根据 Issue 内容生成。