agentd should warn that -t does not work for userparameters

it was concluded today on irc that until zabbix_agentd -t method works for userparametrs, help output should mention that it does not work for userparameters.
i could do that, but i'd like to get 'ack' on that first, maybe the fix is already ready.

text could be something like this :
" Note that -t and -p switches do not work with user parameters. Use zabbix_get instead." (could be longer, but i was aiming for a short version here).

this problem has mislead too many people already