[css-tables] Clarify that cells have their width imposed when doing row layout

[FremyCompany]

The minimum height of a row (without spanning-related height distribution) is defined as the height of an hypothetical linebox containing the cells originating in the row and where cells spanning multiple rows are considered having a height of 0px (but their correct baseline). In this hypothetical linebox, cell heights are considered auto, but their other properties are conserved.

This doesn't mention that the width of table-cells is imposed by the column sizes determined in the previous section. It should. (detected while re-reading the text to answer #4033).

[FremyCompany]

It is important because it changes where the lines wrap, which can affect both the total height of the cell, and its baseline (depending on whether a larger inline makes it to the first line or not).

It's somewhat obvious that you need to do that, but it should be in the spec nonetheless.