Behavior spec: Add section about the Create and Offer flows
    
    This new text may seem ambiguous and confusing, and I hope it can
    improve, but the purpose of it to remove those verbose details from the
    description of how to open a ticket, allowing that text to be much more
    concise.
    
    zPlus, you said it's too complicated and will scare away potential
    implementors. In the next commit I'll try to simplify it and remove all
    the detail now explained in the new section I just added, and let's see
    how it goes ^_^