Tweak description of `change_display_resolution` CVAR

[MrAlaux]

What do you think?

While we're on it, and to make this PR more worthwhile, I'd like to say that the rest of descriptions could probably use a revision; some start with uppercase (e.g. "Widescreen [...]") and some with lowercase (e.g. "vertical resolution [...]"); some indicate the default value despite the fact that the prefix showing the possible values already does it (e.g. the menu_backdrop description), and some don't.

If you'd like, we could settle on those points and I could revise the descriptions myself.