[csswg-drafts] [css-grid] Example 4 from specification (#3410)

Gunyon has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-grid] Example 4 from specification ==
I try to learn css-grid and there is a confusing situation in the example 4 form the [specs](https://www.w3.org/TR/css-grid-1/#overview-placement).

These expressions do not seem to be equivalent

`grid-area: header-start / sidebar-start / footer-end / sidebar-start;`

`grid-row: header-start / footer-end;  grid-column: sidebar-start / footer-end;`

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/3410 using your GitHub account