[css-grid] grid-area propdef doesn't explicitly list longhands

[Loirooriol]

https://drafts.csswg.org/css-grid/#propdef-grid-area doesn't explicitly list the longhands of the grid-area shorthand.

It's not that bad because they can be guessed from the description and appear in the (informative) table in https://drafts.csswg.org/css-grid/#common-uses

But I think it would be more clear to explicitly list the longhands like https://drafts.csswg.org/css-grid/#propdef-grid-row does, e.g. add

The grid-area property is a shorthand for grid-row-start, grid-column-start, grid-row-end and grid-column-end.