Just as a casual test... I opened up Microsoft Word (online version). There's a button on the Review ribbon labelled "Spelling & Grammar", click that and the realtime, inline suggestions and squiggles are off no matter how many errors are present... click that button again and they're there.

So there are two modes... and have been for as long as I can remember (maybe since automatic spell check was there) and it is just a button press.

Now knowing that it's there... well... how many people review feature documentation these days, especially for something that is "feature rich", like Word?