Improve the error message displayed when using a standalone macro in inline context

Users frequently don't understand what this means and we've had to create a FAQ entry: http://www.xwiki.org/xwiki/bin/view/FAQ/Why+do+I+get+a+%22Not+an+inline%22+macro+error

It would be better to explain it in a nicer way in the generated error so that this FAQ entry is not necessary any more. An alternative would be to move this FAQ entry in the rendering wiki somewhere and add a link to it in the generated error message.