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).

Start by running the provided rg scan under docs\xml, then inspect the matching files in Android.Telecom and ns-Android.Telecom.xml. Replace the 626 "To be added" placeholders with accurate XML documentation, using existing completed entries and the related documentation PRs as references. Done means the scan finds no remaining placeholders in this namespace, excluding docs\xml\index.xml.

Written by the indexing model from the issue text.