Add XML documentation for Android.Net.Vcn

The `Android.Net.Vcn` namespace contains **296** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **294** to **296** (increased by **2**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to inspect the 296 `To be added` placeholders under Android.Net.Vcn, including the namespace XML file. Replace the placeholders with XML documentation for the affected API entries, then rerun the scan and confirm no matching placeholders remain.

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