Javadocs do not describe everything available in a library. Most libraries include private classes, methods, fields, and constructors reserved for internal use. Using skript-mirror, these internal APIs are accessible just like any public API.
Usually, private APIs are private for a reason! Make sure you know what you're doing before you start digging around!
skript-mirror has built-in tools for dumping all of the available members of an object. If you need a list of these members, including their return types and input parameters, you can use the Members expression. If you only need a list of names, you can use the Member Names expression.
The best way to learn about how a library works is to read the source code! Many libraries will have their source code easily available online, though you may have to resort to decompiling libraries that are not open source.