The CoreMap key identifying the annotation's true-cased text.
Note that this key is intended to be used with many different kinds of
annotations - documents, sentences and tokens all have their own text.
Returns the type associated with this annotation. This method must
return the same class type as its value type parameter. It feels like
one should be able to get away without this method, but because Java
erases the generic type signature, that info disappears at runtime.