Add XML documentation for Java.Nio.Channels

The `Java.Nio.Channels` namespace contains **417** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **416** to **417** (increased by **1**). Related documentation PRs: [#405](https://github.com/dotnet/android-api-docs/pull/405), [#419](https://github.com/dotnet/android-api-docs/pull/419), [#606](https://github.com/dotnet/android-api-docs/pull/606), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg scan to enumerate the 417 placeholders under docs/xml/Java.Nio.Channels and in ns-Java.Nio.Channels.xml. Read the surrounding API entries and related documentation PRs (#405, #419, #606, and #644) first; done means the namespace's reported “To be added” count is cleared without changing docs/xml/index.xml.

索引モデルが issue の本文から書いたものです。