[KCMs/Night Color] Explain Night Color feature inline
Summary:
All the other platforms that have implemented this feature offer a little inline
explanation of what it is and what it does, but we do not. I think it makes sense to add
our own, since it's not a totally intuitive thing.
Test Plan:
Reviewers: VDG, Plasma, ndavis, zzag
Reviewed By: VDG, Plasma, ndavis, zzag
Subscribers: ahiemstra, davidedmundson, ndavis, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D24906