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).

Search docs\xml with the provided PowerShell rg command, focusing on Java.Util.Concurrent/** and ns-Java.Util.Concurrent.xml while excluding docs\xml\index.xml. Review the matching source XML documentation entries and replace the “To be added” placeholders with documentation. Done means the 1,008 placeholders in this namespace have been addressed.

Written by the indexing model from the issue text.