Improve Async and Sync documentation

[ioquatix]

Summary

• Add a JavaScript async/await comparison for Async and Sync.
• Explain that Async returns a task while Sync returns the block value directly.
• Clarify that Ruby needs an event loop from a fiber scheduler, unlike JavaScript where the event loop is always present.

Discussion: #285 (reply in thread)

Testing

• Not run; documentation-only change.