Specific to Java two sources I've enjoyed are the Java JavaDoc HowTo http://www.oracle.com/technetwork/java/javase/documentation/index-137868.html

And Item 44: Write doc comments for all exposed API elements from Effective Java 2nd Edition.

Specific to Java one source I've enjoyed is Oracle's How to Write Doc Comments for the Javadoc Tool:

This document describes the style guide, tag and image conventions we use in documentation comments for Java programs written at Java Software, Sun Microsystems.

And Item 44: Write doc comments for all exposed API elements:

If an API is to be usable, it must be documented. Traditionally API documentation was generated manually, and keeping it in sync with code was a chore. The Java programming environment eases this task with the Javadoc utility. Javadoc generates API documentation automatically from source code with specially formatted documentation comments, more commonly known as doc comments.

from Effective Java 2nd Edition.