[KCMs/Workspace] Add explanatory labels for click-related settings

Summary: This way the behavior of the related action is clear no matter which setting is chosen.

Reviewers: #vdg, #plasma, Fuchs, ndavis

Reviewed By: #vdg, Fuchs, ndavis

Subscribers: davidedmundson, ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25873