You may have browsed JavaFX API docs generated by javafxdoc tool. javafxdoc tool is implemented as a doclet. It is possible to use javafxdoc's doclet to generate API docs for Java code.
Note: The XMLDoclet implemented as part of javafxdoc tool is an implementation detail and not part of official JavaFX/tool API and so please do not depend on this. This is more for fun/learning/personal use!)
I generated javadoc for BTrace source using the following command:
javadoc -docletpath $JAVAFX_HOME/lib/shared/javafxdoc.jar -doclet com.sun.tools.xmldoclet.XMLDoclet -sourcepath . -subpackages com.sun.btrace
where JAVAFX_HOME is the directory where JavaFX SDK lives.
A screenshot from generated docs: