I'm afraid that workflow event type parameters are not documented anywhere. The easiest way to find what parameters you have access to is to enable the DebugOutput and DebugRedirection and to put something like the following code to display the structure of the parameter_list in the debug output :
function execute( $process, $event )
{
$parameters = $process->attribute( 'parameter_list' );
eZDebug::writeDebug( $parameters, __METHOD__ );
// your code here
}