The FreeBSD Documentation Project: fast build?

I have the documentation project ports and source files installed. I have previously done a full build of the documentation. Is there a setting to tell make to not build the entire document set once everything is built? To build just the stuff that has changed? Similar to NO_KERNCLEAN and NO_CLEAN for make buildworld and make kernel?
Not really, no. Building inside a language directory only builds that language, though. And the same can be done for HTML builds with command-line variables.
Although it depends on what you are doing. Are you building the documentation just to have the latest version? Because the HTML and PDF versions on the site are rebuilt hourly. Or are you trying to do something else, like edit documents and rebuild them?