Various small changes: comments, renamed methods and improved statements

[ramzesenok]

Improved the way comments are written:

• overrides don't need description comments, they ignore them
• description comments are used only to describe the next variable/function/class/etc
• variables don't return values, they store them instead, therefore it's not absolutely correct to describe them as /// Returns something. I didn't change it everywhere, there are a lot, only a few. (IMO even computed properties are still variables, their computation is incapsulated)
• // MARK: Blah-blah should be // MARK: - Blah-blah. This is nice and it also helps Xcode (it will create a nice line in the code editor to help you visually separate things)

Renamed few methods to more suitable names

• cell.correctAnswer(), cell.wrongAnswer() and cell.unknownAnswer() are not really saying immediately what are they doing, so I guess cell.setCorrect/Wrong/UndefinedAnswerStyle() suits better

Improved some statements

• switch-case instead of if where it seemed better
• Other tiny changes

[twostraws]

Thank you for your contribution! It's generally better to submit several smaller pull requests that address specific issues, rather than submit one thing that does a lot at once. In this case, I don't wish to accept your change from triple to double comments, because I think it does more harm than good, but I can't accept the others until you with draw those.

It doesn't really matter whether triple-slashed comments are ignored or not; they are used consistently before things to document what they do. If some things are double-slashed and others tripled, it raises questions that really aren't interesting.

[twostraws]

Great – thank you!