gvariant: Add example to docs for g_variant_builder_open()
gvariant: Add example to docs for g_variant_builder_open()

Try to clarify that the type is the type of the container, not of the
items with in it; and give an example of how to use it for nested types.

https://bugzilla.gnome.org/show_bug.cgi?id=777592

johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib