[css-tables] Clarify row layout (fixes #4121)

Author: FremyCompany

css-tables-3/Overview.bs

@@ -1920,6 +1920,7 @@ spec:css-sizing-3; type:property; text:box-sizing
 				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,
+				their width (including borders and paddings) is forced to the widths and inner spacings of the columns they span,
 				but their other properties are conserved.
 
 			For the purpose of calculating this height,