gtkcontainer: clarify requirements for implementations
gtkcontainer: clarify requirements for implementations

Spent quite a few time investigating why the widgets were
not shown in my GtkContainer implementation.

https://bugzilla.gnome.org/show_bug.cgi?id=758087

grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita