DocTrees should provide getType(DocTreePath) method

The DocTrees API currently provides a getElement(DocTreePath) method that allows to retrieve the javax.lang.model.element.Element referred to by a DocTree node.

However, the element API cannot represent all types. For JDK-8177280 we need a way to represent instantiated generic types with their type arguments. For this purpose we should provide a method that returns a TypeMirror for the type referred to by a DocTreePath, or null if it does not refer to a type.

    TypeMirror getType(DocTreePath path)