[css-grid] Clarify resolved value of track listing is only special for grid containers

[Loirooriol]

From https://drafts.csswg.org/css-grid/#resolved-track-list,

When an element’s display is grid or inline-grid and it generates a box, the resolved value of the grid-template-rows and grid-template-columns properties is the used value

But the condition doesn't specify whether it's talking about the computed or the used value of display. This matters because a computed value of display: grid may not produce a grid container, e.g. for replaced elements like <svg>, or for magical elements like <details>.

So I think it should be made explicit that it's the used value of display, or just say

When an element generates a grid container box, ...