Add XML documentation for Android.Net.Sip

The `Android.Net.Sip` namespace contains **67** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **74** to **67** (reduced by **7**). Related documentation PRs: [#612](https://github.com/dotnet/android-api-docs/pull/612), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command against docs\xml to enumerate the 67 remaining To be added placeholders under Android.Net.Sip. Update the corresponding source XML documentation, then rerun the scan; done means the reported placeholders are addressed, excluding docs\xml\index.xml.

Vom Indexierungsmodell aus dem Issue-Text verfasst.