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.

Written by the indexing model from the issue text.