Add Smart Contract Function Documentation

[LawalRahman]

Add comprehensive JSDoc comments to all Soroban smart contract functions in Rust:

• payment_channel.rs: Document all public functions, parameters, return values
• escrow.rs: Document escrow logic and state transitions
• merchant_registry.rs: Document merchant operations
• recurring_payment.rs: Document subscription logic

Include example invocations for each function.

Acceptance Criteria:

• All public functions documented
• Parameter types and meanings explained
• Example invocations provided
• Build succeeds without warnings