You don't have to memorise the syllabus in any capacity. All it means is that you should know what you can be assessed on and what you should be learning/take away from the subject. You should also know where it's worth extending the depth of your knowledge (ie. where to flex on markers and effectively signpost your marks). Some people prefer to essentially take notes on the syllabus; it's not necessary to do this but if it helps you set out your aims better or whatever for the next 12 months, go for it. At the other end of the spectrum, it's possible to achieve high marks without looking at the syllabus once, though somewhat more difficult.
You're not going to get docked marks for not reflecting the syllabus. Not once have I seen a marking scheme explicitly mention 'student reflects syllabus'. What you will get docked marks for (for a subject like Modern History) is not having coherent and concise arguments, etc - read marking criteria for past assessments! Of course, you have to reflect the syllabus to some extent because otherwise, you're just spouting BS.
Hopefully this clears things up somewhat