doc: Adapt GtkPackType documentation to GTK4 changes
Since GTK4, the functions that used the GtkPackType enumeration,
gtk_box_pack_{start,end}
, were replaced by gtk_box_{append,prepend}
,
hence this enumeration isn't used anymore by any function within the
GtkBox type, and the child packing properties were also removed for GTK4.
So this commit adapts the documentation accordingly.