[css-grid] Fill in missing detail on how auto-fill works for line-name-lists.

Author: fantasai

css-grid/Overview.bs

@@ -1182,6 +1182,9 @@ Repeating Rows and Columns: the ''repeat()'' notation</h4>
 		but the same <a>track list</a> can also contain <<fixed-repeat>>s.
 	* The <<name-repeat>> variant can be used with the ''grid-template-rows/subgrid'' keyword:
 		it only repeats names.
+		The ''auto-fill'' keyword is only valid once per <<line-name-list>>,
+		and repeats enough times for the name list to match the subgrid’s specified <a>grid span</a>
+		(defaulting to 0 if that is ''auto'' or if the span is already fulfilled).
 
 	<div class='example'>
 		This example shows two equivalent ways of writing the same grid definition.
@@ -1424,6 +1427,8 @@ Subgrids: the ''grid-template-rows/subgrid'' keyword</h4>
 			it also automatically receives the line names specified for its parent <a>grid</a>.
 			(In such cases the author can rely on the names specified in the parent grid,
 			and does not need to duplicate those names in each subgrid declaration.)
+
+			Issue: People should review how subgrid line names are assigned, to ensure it is sane and useful.
 	</ul>
 
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