Specify for each (optional) parameter, whether it is OUT or INOUT
Per https://gi.readthedocs.io/en/latest/annotations/giannotations.html the (optional) annotation is for (out) or (inout) parameters. Update:
gio/gdbusconnection.h:492: * @out_user_data: (nullable) (not optional): Return location for user data to pass to functions in the returned #GDBusInterfaceVTable (never %NULL).
gio/gvdb/gvdb-reader.c:335: * @length: (optional): the number of items returned, or %NULL
glib/gtree.c:856: * @orig_key: (optional) (nullable): returns the original key
glib/gtree.c:857: * @value: (optional) (nullable): returns the value associated with the key
glib/gvarianttypeinfo.c:224: * @alignment: (optional): the location to store the alignment, or %NULL
glib/gvarianttypeinfo.c:225: * @fixed_size: (optional): the location to store the fixed size, or %NULL
glib/gvarianttypeinfo.c:333: * @alignment: (optional): the location to store the alignment, or %NULL
glib/gvarianttypeinfo.c:334: * @fixed_size: (optional): the location to store the fixed size, or %NULL
to state OUT or INOUT.