Add XML documentation for Android.Credentials

The `Android.Credentials` namespace contains **454** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **434** to **454** (increased by **20**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command in docs\xml to enumerate the 454 matching placeholders under Android.Credentials. Review the matching source XML documentation entries and replace each placeholder with appropriate documentation, excluding docs\xml\index.xml. Done means the scoped scan no longer reports those placeholders.

由索引模型根据 Issue 内容生成。