Grammar and phrasing fixes

Fixes various grammar and phrasing issues in the spec. Also fixes a seeming typo in the restriction of discriminator property value of models to be the names of the parent model or any of its sub-models. The intent seems to restrict the discriminator property value of models to NOT be the name of the parent model or any of its sub-models. (Let me know if that wasn't the intent and I can revert that change).
The original statement is correct. That's what the discriminator is used for.
Say you have these models (simplified sample):
Pet (discriminator:type) { type, name, age }
Cat extends Pet { hasClaws }
Dog extends Pet { likesCats }

In that case, the 'type' field has to be either Pet, Cat or Dog (the model IDs) to indicate which model it is. It can't be any value as that won't be able to be parsed by the consumer/producer.