Add XML documentation for Android.AdServices.CustomAudiences

The `Android.AdServices.CustomAudiences` namespace contains **465** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **460** to **465** (increased by **5**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command in docs\xml to inspect the 465 remaining placeholders under Android.AdServices.CustomAudiences and the matching ns-Android.AdServices.CustomAudiences.xml file. Review the surrounding XML documentation entries to determine the required wording, then confirm completion by rerunning the scan with no remaining 'To be added' placeholders.

Rédigé par le modèle d'indexation à partir du texte de l'issue.