Since the appearance of reprs is not guaranteed.  I don't think this should be documented (because doing so makes it a guaranteed behavior).  In fact, the appearance has changed over time from ... to  [...] in and may change at some point to <...> so that it can't be confused with an actual Ellipsis object.  At best this should be a FAQ entry or we can defer to behaviors that are documented and guaranteed (such as reprlib.recursive_repr() and its default fillvalue).  

I concur with David Murray that the Ellipsis object should be indexed and cross-referenced.