Move ExtendedCursor::Shape's doc comment to the correct place

Summary:
It probably has to be placed above the enum, though I'm not sure whether
it applies to ExtendedCursor::Shape or CursorShape.

Test Plan: Compiles.

Reviewers: #kwin, apol

Reviewed By: apol

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18669