[css-grid-2] Clarify auto margins on subgrids

[fantasai]

https://www.w3.org/TR/css-grid-2/#subgrid-box-alignment

The subgrid is always stretched in its subgridded dimension(s): the align-self/justify-self properties on it are ignored, as are any specified width/height constraints.

We should clarify that auto margins resolve to zero.