Add XML documentation for Android.Telecom

The `Android.Telecom` namespace contains **626** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **1,142** to **626** (reduced by **516**). Related documentation PRs: [#538](https://github.com/dotnet/android-api-docs/pull/538), [#548](https://github.com/dotnet/android-api-docs/pull/548), [#632](https://github.com/dotnet/android-api-docs/pull/632), [#644](https://github.com/dotnet/android-api-docs/pull/644), [#666](https://github.com/dotnet/android-api-docs/pull/666).