Use request.remote_ip instead of request.ip to obtain buyer IP

[aminzam]

Description

This is a usage doc update to guide devs to accurately obtain the buyer IP using rail's request.remote_ip rather than request.ip. This is because the buyer can be behind a proxy server. The proxies usually set HTTP_CLIENT_IP and/or HTTP_X_FORWARDED_FOR headers to track the originating client IP. These headers are properly handled by request.remote_ip (doc). The developer can optionally introduce a set of known proxy servers to further control this behaviour by setting Rails.application.config.action_dispatch.trusted_proxies.

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation.
• I have added a changelog line.