Add XML documentation for Android.OS.Health

The `Android.OS.Health` namespace contains **109** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **107** to **109** (increased by **2**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start in docs\xml and run the provided rg command to list the 109 `To be added` placeholders under Android.OS.Health, excluding docs\xml\index.xml. Review the matched source XML documentation entries and replace the placeholders with the required API documentation. Done means the scan no longer reports those Android.OS.Health placeholders.

索引モデルが issue の本文から書いたものです。