Add XML documentation for Android.AdServices.AdSelection

The `Android.AdServices.AdSelection` namespace contains **773** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **757** to **773** (increased by **16**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to enumerate the 773 placeholders under Android.AdServices.AdSelection, then inspect the matching namespace XML files and nearby API context. Replace the relevant “To be added” entries with accurate documentation; done means the scoped scan reports no remaining placeholders, excluding docs\xml\index.xml.

Written by the indexing model from the issue text.