[css-grid] Clarify what to do when even one repetition overflows.

Author: tabatkins

css-grid/Overview.bs

@@ -1228,7 +1228,9 @@ Repeating Rows and Columns: the ''repeat()'' notation</h4>
 	that does not cause the <a>grid</a> to overflow its <a>grid container</a>
 	(treating each track as its <a>max track sizing function</a> if that is <a>definite</a>
 	or as its minimum track sizing function otherwise,
-	and taking 'grid-gap' into account).
+	and taking 'grid-gap' into account);
+	if any number of repetitions would overflow,
+	then 1 repetition.
 	Otherwise, if the <a>grid container</a> has a <a>definite</a> min size in the relevant axis,
 	the number of repetitions is the smallest possible positive integer that fulfills that minimum requirement.
 	Otherwise, the specified <a>track list</a> repeats only once.