[Wallpaper Configuration] Reword inline message concerning widget locking

Summary: Widget locking is now only a hidden parameter that can be set up as a restriction so we're rewording the inline message to take that into account.

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: mart, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29366