Documentation update (transforms, key maps, isNew, embedded)

Should take the place of #496, and goes further.

* Update the custom transforms documentation.
* Update how to set a custom key for an attribute.
* State that embedded data is not supported after revision 5. (I'm happy to be wrong about this- but the docs are missing or this isn't supported yet. See #504 for an example of the confusion.)
* Document `isNew` as a state on Ember data models.

Happy to squish if that's how y'all prefer your documentation updates served. I wanted the commits for now on the chance that anything should be ripped out or altered.