The official Java documentation is filled with long paragraphs that describe classes and methods. How can these be produced using javadoc?

In the Java documentation creator, there's no limit to the length of a description. Although they're often as brief as a sentence or two, they can be longer if necessary. End the description with a period, immediately followed by a new line with a tag of some kind or the end of the comment.

Do I have to document everything in my Java classes if I'm planning to use the javadoc tool?

The Java documentation creator will work fine no matter how many or how few comments you use. Deciding what elements of the program need to be documented is up to you. You probably should describe the class and all methods, variables, and objects that aren't hidden from other classes. The javadoc tool will display a warning each time a serializable object or variable is defined in a program without a corresponding Java documentation comment.