Add XML documentation for Android.Ranging.Ble.Rssi

The `Android.Ranging.Ble.Rssi` namespace contains **47** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **45** to **47** (increased by **2**). 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 47 `To be added` placeholders under Android.Ranging.Ble.Rssi and the ns-Android.Ranging.Ble.Rssi.xml file. Review the surrounding API declarations and existing documentation patterns, then confirm the scan finds no remaining placeholders in this namespace.

Written by the indexing model from the issue text.