Add XML documentation for Java.Time.Chrono

The `Java.Time.Chrono` namespace contains **365** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Java.Time.Chrono/**' --glob 'ns-Java.Time.Chrono.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **380** to **365** (reduced by **15**). Related documentation PRs: [#467](https://github.com/dotnet/android-api-docs/pull/467), [#486](https://github.com/dotnet/android-api-docs/pull/486), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg scan across docs\xml, then inspect the Java.Time.Chrono directory and ns-Java.Time.Chrono.xml. Replace the reported 365 “To be added” placeholders with accurate XML documentation, excluding docs\xml\index.xml; rerun the scan to confirm none remain.

Rédigé par le modèle d'indexation à partir du texte de l'issue.