JavaDoc is a tool to explain to the public how to access the "application
public interface".
If the public can not get at the class - it will not be documented in the API.
From
javadoc - The Java API Documentation Generator.
The JavadocTM tool parses the declarations and documentation comments in a set of Java source files and produces a corresponding set of HTML pages describing (by default) the public and protected classes, nested classes (but not anonymous inner classes), interfaces, constructors, methods, and fields.