QQC2 Styles style labels themselves, we don't to do this via a
subclass.
This is a relic from Kirigami 1 where labels were not themed, and whilst
it makes some sense for source compatibility, encouraging its use does
not; Both the gallery and systemsettings already has a weird mix of both going on.
This patch also adjusts documentation that talks about Plasma styles;
which I think is a copy-paste error.