container: Slightly reword some docs
container: Slightly reword some docs

Clarify gtk_container_remove documentation regarding reference
holding. Suggested in

https://bugzilla.gnome.org/show_bug.cgi?id=757607

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