[MRG] DOC Refactor example to remove ambiguity in understanding param order

Fixes #3267
Oh sorry I misunderstood the dict we are talking about. I see the issue now. But I don't think your PR actually fixes that. The ``__iter__`` mechanism enforces that the alphabetically last parameter gets varied first.
It produces a list of dicts, and this issue is how are the items ordered in each entry of the list, which is not guaranteed to be even consistent over the list.