man: update struct io_uring_buf_reg documentation

Was missing the flags field, and also the newly added 'min_left' field
that can be used with incrementally consumed buffers to set a minimum
number of bytes that should be left in a buffer for it to still be
considered valid. If less than 'min_left' is left it'll be marked as
done (IORING_CQE_F_BUF_MORE not set), and the next consumption point
will be the next buffer in that group.

Signed-off-by: Jens Axboe <axboe@kernel.dk>