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).
Please verify that the change to the last sentence is the actual intent of the spec, since it changes a MUST to a MUST NOT (effectively changing the spec).