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.

Written by the indexing model from the issue text.