Fix supported OS attributes for api.kde.org

Summary:
Move the "maintainer needed" to a note, so it's not showing up globally
for all modules. Also, change "macOS" back to "MacOSX" (despite technically
being the right name), as every other module still uses MacOSX and thus
using anything else essentially adds a new OS on api.kde.org, globally.

Reviewers: Frameworks, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D23188