Package-level javadoc comments are placed in a file named package-info.java inside the package directory. It contains the comment and a package declaration:
/**
* Provides the classes necessary to create an applet and the classes an applet uses
* to communicate with its applet context.
* <p>
* The applet framework involves two entities:
* the applet and the applet context. An applet is an embeddable window (see the
* {@link java.awt.Panel} class) with a few extra methods that the applet context
* can use to initialize, start, and stop the applet.
*
* @since 1.0
* @see java.awt
*/
package java.lang.applet;
This is documented here: Package Comment Files