Add XML documentation for Java.Time.Format

The `Java.Time.Format` namespace contains **0** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **230** to **0** (reduced by **230**). Related documentation PRs: [#450](https://github.com/dotnet/android-api-docs/pull/450), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by rerunning the provided rg scan against docs\xml for Java.Time.Format and ns-Java.Time.Format.xml. The update log reports zero remaining placeholders, so the work is complete when the scan still returns no matches; otherwise, inspect the matching XML documentation entries.

索引モデルが issue の本文から書いたものです。