docs: Add IIFE pattern for return statements in JS rules

[ammar-agent]

Addresses #87 by documenting how to use return statements in JavaScript rules via the IIFE pattern.

Changes

• Added "Using Return Statements" section to JavaScript rules documentation
• Shows simple IIFE example with return statements

Testing

• Verified IIFE pattern works with manual testing
• All existing tests pass
• No code changes, documentation only

This keeps the implementation simple while giving users the capability they requested.