Documentation for `gt_bboxes` argument of `mask_head_loss_pre` function is missing

[mitmul]

The function mask_head_loss_pre takes gt_bboxes as the 4th positional argument, but the docstring does not contain what this argument is. https://github.com/chainer/chainercv/blob/master/chainercv/links/model/fpn/mask_head.py#L171