The "documented information" you want is mentioned in the article. Guido links to a blog post by Jeremy Siek and a paper by Vitousek, Siek and Baker. Read the blog post, section "Why subtyping does not work".

http://wphomes.soic.indiana.edu/jsiek/what-is-gradual-typing...

http://wphomes.soic.indiana.edu/jsiek/files/2014/03/retic-py...