doc: Add a note about prioritization of GtkStyleProviders
doc: Add a note about prioritization of GtkStyleProviders

It was unclear in the docs what happened if
gtk_style_context_add_provider() and
gtk_style_context_add_provider_for_screen()
were used with the same priority.

Distrotech/gtk2,chergert/gtk,jadahl/gtk,chergert/gtk,davidgumberg/gtk,Distrotech/gtk2,Lyude/gtk-,Lyude/gtk-,grubersjoe/adwaita,jadahl/gtk,jessevdk/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,Adamovskiy/gtk,chergert/gtk,chergert/gtk,grubersjoe/adwaita,Lyude/gtk-,grubersjoe/adwaita,msteinert/gtk,davidgumberg/gtk,Lyude/gtk-,Lyude/gtk-,jessevdk/gtk,davidt/gtk,alexlarsson/gtk,jadahl/gtk,Lyude/gtk-,jadahl/gtk,Sidnioulz/SandboxGtk,simokivimaki/gtk,grubersjoe/adwaita,jigpu/gtk,jessevdk/gtk,ahodesuka/gtk,alexlarsson/gtk,Distrotech/gtk2,grubersjoe/adwaita,bratsche/gtk-,jigpu/gtk,ahodesuka/gtk,ebassi/gtk,ebassi/gtk,davidgumberg/gtk,Adamovskiy/gtk,Adamovskiy/gtk,alexlarsson/gtk,ahodesuka/gtk,chergert/gtk,jigpu/gtk,ahodesuka/gtk,chergert/gtk,ebassi/gtk,alexlarsson/gtk,Adamovskiy/gtk,Distrotech/gtk2,jadahl/gtk,Adamovskiy/gtk,bratsche/gtk-,davidt/gtk,davidt/gtk,chergert/gtk,Lyude/gtk-,Distrotech/gtk2,ebassi/gtk,jigpu/gtk,bratsche/gtk-,bratsche/gtk-,simokivimaki/gtk,ahodesuka/gtk,davidgumberg/gtk,ebassi/gtk,jessevdk/gtk,jigpu/gtk,jigpu/gtk,jessevdk/gtk,alexlarsson/gtk,bratsche/gtk-,Sidnioulz/SandboxGtk,simokivimaki/gtk,ebassi/gtk,davidt/gtk,ahodesuka/gtk,bratsche/gtk-,davidgumberg/gtk,simokivimaki/gtk,ahodesuka/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,ahodesuka/gtk,jessevdk/gtk,chergert/gtk,jadahl/gtk,msteinert/gtk,jigpu/gtk,davidt/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,Adamovskiy/gtk,msteinert/gtk,Lyude/gtk-,davidgumberg/gtk,jadahl/gtk,davidgumberg/gtk,Adamovskiy/gtk,grubersjoe/adwaita,Distrotech/gtk2,msteinert/gtk,simokivimaki/gtk,Sidnioulz/SandboxGtk,jadahl/gtk,jigpu/gtk,msteinert/gtk,alexlarsson/gtk,Adamovskiy/gtk,davidt/gtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,simokivimaki/gtk