Add XML documentation for Android.Ranging.Wifi.Rtt

The `Android.Ranging.Wifi.Rtt` namespace contains **129** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **74** to **129** (increased by **55**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the XML files under docs\xml matching Android.Ranging.Wifi.Rtt/** and ns-Android.Ranging.Wifi.Rtt.xml, then run the provided ripgrep command to inspect all 129 placeholders. Replace the matching “To be added” documentation with appropriate API descriptions and rerun the scan, with no remaining placeholders in that namespace except docs\xml\index.xml.

Written by the indexing model from the issue text.