docs: Explain 'icon theme context' better in gtkicontheme.c
docs: Explain 'icon theme context' better in gtkicontheme.c

Add links from gtk_icon_theme_list_contexts() to
gtk_icon_theme_list_icons(), and from there to the Icon Theme
Specification and the Icon Naming Specification.

https://bugzilla.gnome.org/show_bug.cgi?id=461249

jadahl/gtk,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,Lyude/gtk-,davidgumberg/gtk,Adamovskiy/gtk,chergert/gtk,ahodesuka/gtk,chergert/gtk,chergert/gtk,davidgumberg/gtk,chergert/gtk,jadahl/gtk,Lyude/gtk-,alexlarsson/gtk,jadahl/gtk,grubersjoe/adwaita,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,Lyude/gtk-,ahodesuka/gtk,ahodesuka/gtk,jigpu/gtk,Lyude/gtk-,grubersjoe/adwaita,chergert/gtk,jadahl/gtk,Adamovskiy/gtk,jigpu/gtk,alexlarsson/gtk,Lyude/gtk-,chergert/gtk,grubersjoe/adwaita,ahodesuka/gtk,jigpu/gtk,ahodesuka/gtk,jadahl/gtk,davidgumberg/gtk,Adamovskiy/gtk,Adamovskiy/gtk,Lyude/gtk-,jadahl/gtk,davidgumberg/gtk,Lyude/gtk-,chergert/gtk,alexlarsson/gtk,ahodesuka/gtk,Adamovskiy/gtk,davidgumberg/gtk,alexlarsson/gtk,ahodesuka/gtk,alexlarsson/gtk,alexlarsson/gtk,Adamovskiy/gtk,jadahl/gtk,jigpu/gtk,grubersjoe/adwaita,davidgumberg/gtk,grubersjoe/adwaita,davidgumberg/gtk,ahodesuka/gtk,Adamovskiy/gtk,davidgumberg/gtk,Adamovskiy/gtk,jigpu/gtk,Lyude/gtk-,grubersjoe/adwaita,jigpu/gtk