According to step 5.3 of the manual you posted:
it appears that you push the cover up and turn to the right (clockwise) with 1.91kg of force (which is very specific, but doesn't strike me as much, but I've not seen force noted that way before) to install the cover*. That implies that to remove the cover, you rotate to the left (counter/anti-clockwise), then pull down.
This appears to be borne out by step 1.2 on the furthest right panel of the instructions.
If you'll note the section of the instructions I've highlighted:
There appears to be an angled slot that will pull the lamp cover up into place as it is turned and lower it away from a key on the base as it is removed. You may need to pull down very slightly as you turn it.
Also, note that in the instructions, there is an arrow on the base (screwed into the ceiling) that points down, and an arrow on the lamp cover that points up. In the instructions snippet I highlighted above, you'll note that if you turn the lamp cover too far that's indicated as being a bad thing. Your pictures of the fixture only show the arrow pointing down - it's possible that the previous installer turned the lamp cover too far and that the lamp cover is now somewhat jammed and will require greater force than was originally intended to remove. Have you looked around the lamp cover to see if there is a line-up arrow on it that isn't properly lined up?
It's also possible that the lamp cover has been in place for a while. Since you don't know how to remove it, I'm assuming that this is the first time you've changed a bulb in this fixture. If it's been there a while, it's possible that there's just been an accumulation of crud built up around the joint between the cover and the base and that it's just a bit sticky and will require a bit of extra force or some wiggling and gentle prying to remove. (Note - the fixture does look very clean, but that doesn't mean there isn't some small stuff that has settled into the joint line that's causing it to stick.)
In general, I think that it's just going to take more force to unscrew the cover than you've been applying.
- If you're renting, have the landlord come take care of changing the light bulb - if he asks, indicate that it's stuck and you don't want to be responsible for breaking it.
- If you own, keep applying force until something happens. Either it will come off and you'll know how hard you have to turn it, or it will break and you'll know that it probably wasn't installed properly. If it breaks, you already know where to go to buy a replacement! Either way, you'll end up with a working light bulb in the fixture. If you don't you'll have a non-working fixture that will annoy you for the rest of the time you live there.
* It just occurred to me - maybe the warning is that the lamp cover weighs 1.91Kg. That would be a very heavy cover and would most certainly warrant a warning as it looks like very light plastic that maybe weighs 100gm.