Documentation of current code is continuously built by the Jenkins job http://qa.nuxeo.org/jenkins/job/nuxeo-doc/.
For releases, run the job manually ("Build Now" link). Then change the job parameters (see parameters' description for details).
This is publishing Javadoc and TLDdoc into http://community.nuxeo.com/api/.
Since Nuxeo version 11.3, this export is automated and run by a Jenkins job.