Add XML documentation for Java.Util.Concurrent

The `Java.Util.Concurrent` namespace contains **1,008** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Java.Util.Concurrent/**' --glob 'ns-Java.Util.Concurrent.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **986** to **1,008** (increased by **22**). Related documentation PRs: [#378](https://github.com/dotnet/android-api-docs/pull/378), [#393](https://github.com/dotnet/android-api-docs/pull/393), [#398](https://github.com/dotnet/android-api-docs/pull/398), [#506](https://github.com/dotnet/android-api-docs/pull/506), [#599](https://github.com/dotnet/android-api-docs/pull/599), [#644](https://github.com/dotnet/android-api-docs/pull/644).

使用提供的 PowerShell rg 命令搜索 docs\xml，重点关注 Java.Util.Concurrent/** 和 ns-Java.Util.Concurrent.xml，同时排除 docs\xml\index.xml。检查匹配的源 XML 文档条目，并将“To be added”占位符替换为文档。当此命名空间中的 1,008 个占位符均已处理后，即视为完成。

由索引模型根据 Issue 内容生成。