Add XML documentation for Java.IO

The `Java.IO` namespace contains **800** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **791** to **800** (increased by **9**). Related documentation PRs: [#371](https://github.com/dotnet/android-api-docs/pull/371), [#403](https://github.com/dotnet/android-api-docs/pull/403), [#562](https://github.com/dotnet/android-api-docs/pull/562), [#626](https://github.com/dotnet/android-api-docs/pull/626), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command first to enumerate the 800 placeholders under docs\xml, then inspect the matching Java.IO/** files and ns-Java.IO.xml entries. Replace the reported “To be added” placeholders with XML documentation, and consider the work complete when the targeted scan no longer reports those placeholders.

由索引模型根据 Issue 内容生成。