document effect of '__traits_version__' entry on __setstate__

I just ran into a nasty bug while doing json-based traits serialization. After restoring my traits, some notifications were broken.

After digging into the code (setstate: https://github.com/enthought/traits/blob/master/traits/has_traits.py#L1427), I noticed it was my fault because I had "cleaned out" the '__traits_version__' entry from the state dictionary. However, the usefulness of this entry is not documented

I propose to add this explicative warning to discourage other people from having the same idea...