Missing API docs in scenegraph classes

There are a few methods, fields, and constructors in various scenegraph classes with missing documentation. There are also a small number in fxml and 1 in base, so I included them here as well.

See the attached file with the list of javadoc warnings for missing comments.

This part of a larger effort to clean up missing or redundant docs, especially as they related to properties. See JDK-8271083 for a general description.