R2-3560: Document listParts and listMultipartUploads on the Workers Binding

[helloimalastair]

Summary

Documenting method options/return values, and adding it to the Multipart example page.

To be merged after cloudflare/workers-sdk#11562 is merged.

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.