[css-display] `display: grid` doesn't establish a GFC for subgrids

[Loirooriol]

A mentioned in #2598 (comment), https://drafts.csswg.org/css-display-3/#valdef-display-grid says

grid
The element generates a principal grid container box, and establishes a grid formatting context.

However, subgrids don't establish a GFC. See https://www.w3.org/TR/css-grid-2/#grid-container

A grid container that is not a subgrid establishes an independent grid formatting context for its contents.

[tabatkins]

Yes, this is true, I suppose we can be a little clearer with subgrid there.