There is no documentation, dtd or schema for the help file syntax, but you can grab the available tags
from looking at the source code of lib/Horde/Help.php.

    var $_handlers = array(
        'help'     =>  '_helpHandler',
        'entry'    =>  '_entryHandler',
        'title'    =>  '_titleHandler',
        'heading'  =>  '_headingHandler',         == "<h2>"..."</h2>"
        'para'     =>  '_paraHandler',            == "<p>"..."</p>"
        'ref'      =>  '_refHandler',             == <ref entry="ahciv-overview" module="ahciv"> (link to a specific help entry?)
        'eref'     =>  '_erefHandler',            == <eref url="http://www.google.com"> (creates an external link)
        'href'     =>  '_hrefHandler',            == <href url="player/player_frame.php" app="ahciv"> (creates an internal link)
        'b'        =>  '_bHandler',               == "<strong>"..."</strong>"
        'i'        =>  '_iHandler',               == "<em>"..."</em>"
        'pre'      =>  '_preHandler',             == "<pre>"..."</pre>"
        'tip'      =>  '_tipHandler',             == "<em class='helpTip'>"..."</em>" (tool tips?)
        'warn'     =>  '_warnHandler'             == "<em class='helpWarn'>"..."</em>" (warning?)
    );