Add XML documentation for Android.Telephony.Ims

The `Android.Telephony.Ims` namespace contains **506** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **511** to **506** (reduced by **5**). Related documentation PRs: [#480](https://github.com/dotnet/android-api-docs/pull/480), [#540](https://github.com/dotnet/android-api-docs/pull/540), [#546](https://github.com/dotnet/android-api-docs/pull/546), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the issue’s rg command in docs\xml and review the matching Android.Telephony.Ims XML files and ns-Android.Telephony.Ims.xml entries. Replace the 506 “To be added” placeholders with XML documentation, then rerun the scan and confirm no matching placeholders remain, excluding docs\xml\index.xml.

Vom Indexierungsmodell aus dem Issue-Text verfasst.