[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: {F7659929}

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