Add XML documentation for Android.Net.Wifi.Rtt

The `Android.Net.Wifi.Rtt` namespace contains **568** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **558** to **568** (increased by **10**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to inspect the 568 placeholders under Android.Net.Wifi.Rtt, excluding docs\xml\index.xml. Review the surrounding API entries to determine the documentation scope for each placeholder. Done means the reported To be added count for this namespace is addressed and the scan confirms no remaining placeholders.

Written by the indexing model from the issue text.