Add XML documentation for Android.Telephony.Gsm

The `Android.Telephony.Gsm` namespace contains **46** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **54** to **46** (reduced by **8**). Related documentation PRs: [#445](https://github.com/dotnet/android-api-docs/pull/445), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg scan in docs\xml to list the 46 remaining “To be added” placeholders under Android.Telephony.Gsm, excluding docs\xml\index.xml. Read the surrounding XML documentation entries and replace each placeholder with appropriate documentation; done means the namespace scan no longer reports those 46 placeholders.

Vom Indexierungsmodell aus dem Issue-Text verfasst.