I was going to write that this was a failing of the Ruby documentation. But actually, it's pretty well documented if you take the time to read it (http://ruby-doc.org/core-2.0/Range.html).
There's absolutely no need to go hitchhiking in the C source in this case.
While I agree that it's documented well enough that once you hit the situation and read the doc you should be able to figure out why without diving into the C, I think the documentation still fails in the sense that it doesn't give you any guidance as to why or when you would choose which one.
That said, I really don't consider this magic as another subthread suggests. include? just has an optimization for numeric ranges, but otherwise behaves exactly as Enumerable#include? should.
There's absolutely no need to go hitchhiking in the C source in this case.