The syntax page is already quite a large page, there is a (valid) concern already discussed (iirc on the mailing list) that adding too much information via plugins would make it even more unwieldy.
At present there is nothing to stop plugin authors including information on their plugin's syntax in the description item of the info array. That field does support multi-line output. Typically I include brief syntax information in that field for my plugins as can be seen http://wiki.jalakai.co.uk/dokuwiki/doku.php/wiki/syntax
Also the current syntax page is relatively simple to translate for those wikis which do not use english. Including all the syntax descriptions in templates would make that more difficult.