Add XML documentation for Java.Util.Jar

The `Java.Util.Jar` namespace contains **387** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **407** to **387** (reduced by **20**). Related documentation PRs: [#468](https://github.com/dotnet/android-api-docs/pull/468), [#473](https://github.com/dotnet/android-api-docs/pull/473), [#504](https://github.com/dotnet/android-api-docs/pull/504), [#507](https://github.com/dotnet/android-api-docs/pull/507), [#618](https://github.com/dotnet/android-api-docs/pull/618), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start in docs\xml, using the provided rg command to inspect the 387 “To be added” placeholders under Java.Util.Jar and ns-Java.Util.Jar.xml. Read the surrounding XML entries to understand the expected documentation style, then replace the placeholders with accurate descriptions. Done means the targeted scan no longer reports those placeholders.

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