The documentation is this article, written by Ron Paul himself.
I quote:
"American citizens would be allowed to spend their hard-earned money wherever they wish across the globe, not told that certain countries are under embargo and thus off limits."
What more do you need than the man's own words in his own opinion piece?
That means that we would not trade freely with nations that do not wish to. Trade embargos would then be appropriate where other nations seek to restrict our access to their markets. That looks pretty well qualified.