The CoreMap key for getting the sentences contained in an annotation.
The sentences are represented as a List<CoreMap>.
Each sentence might typically have annotations such as TextAnnotation,
TokensAnnotation, SentenceIndexAnnotation, and BasicDependenciesAnnotation.
This key is typically set only on document annotations.
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.