Document that handlers (e.g., `TraitType` subclasses) should be stateless

I've seen a couple of instances recently where projects were trying to store per-object state in a `TraitType` instance. That doesn't work, in general: the `TraitType` instance is per-class, not per-object.

In the section of the user docs on writing your own `TraitType` subclass, it should be made clear that `TraitType` instances should be stateless: they shouldn't store per-object state from the `HasTraits` objects using them.