[css-grid] editorial: Grid Container Baselines

[MatsPalmgren]

https://drafts.csswg.org/css-grid/#grid-baseline
"The first/last baselines of a grid container are determined as follows:
If any of the grid items whose areas intersect the grid container’s first/last row/column ..."

I think this chapter would read better if it used "first(last)" style rather than "first/last",
and also made it clearer that the entire text applies independently in each axis,
something like so:

"The first(last) baselines for each axis of a grid container are determined as follows:
If any of the grid items whose areas intersect the grid container’s first(last) track in this axis ..."