Docs: Add rustdoc examples for Request::new_from_response and Captcha::set_remoteip

[gortavoher]

Add concise rustdoc examples: (1) Request::new_from_response showing error handling with ? and no network call, (2) Captcha::set_remoteip demonstrating that empty string clears to None. Acceptance: examples compile as doctests; no API or behavior changes; keep under ~20 lines each.