DOC : modified docs is HasTraits.traits and HasTraits.class_traits

 - change list -> dict to reflect what the code does
 - clarified how kwargs work (instead of just referencing enthought.traits).
 - fix classes -> class' spelling
Merge pull request #5234 from tacaswell/doc_hastraits.traits

DOC : modified docs is HasTraits.traits and HasTraits.class_traits