I did some digging and as it turns out: proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

Adds the $remote_add to the X-Forwarded-For automatically.

So in summary, if X-Forwarded-For is present, then it gets rewritten by nginx
as: X-Forwarded-For: whatever.ip.was.therebeforenginx,the.ip.hitting.nginx

and if no X-Forwarded-For is present, then it gets added by nginx as: X-Forwarded-For: the.ip.hitting.nginx

This behavior is exactly as needed and solves my inquiry. I will update the
wiki to make it more clear that $proxy_add_x_forwarded_for will automatically
include the IP hitting nginx. Thanks.