Add XML documentation for Xamarin.Android.Net

The `Xamarin.Android.Net` namespace contains **123** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **129** to **123** (reduced by **6**). Related documentation PRs: [#555](https://github.com/dotnet/android-api-docs/pull/555), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to list the 123 placeholders under Xamarin.Android.Net and ns-Xamarin.Android.Net.xml. Replace the relevant “To be added” XML documentation entries, then rerun the scan; done means the scoped placeholder count is reduced to zero, excluding docs\xml\index.xml.

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