[css-grid][css-flexbox] Clarify that only the actual content-based minimum size causes the box to be indefinite; if it resolves 0 it has no effect on definiteness.

Author: tabatkins

css-flexbox-1/Overview.bs

@@ -948,7 +948,7 @@ Automatic Minimum Size of Flex Items</h3>
 
 	For the purpose of calculating an intrinsic size of the box
 	(e.g. the box’s <a>min-content size</a>),
-	this value causes the box’s size in that axis to become indefinite
+	a <a>content-based minimum size</a> causes the box’s size in that axis to become indefinite
 	(even if e.g. its 'width' property specifies a <a>definite</a> size).
 	Note this means that percentages calculated against this size
 	will [=behave as auto=].

css-grid-1/Overview.bs

@@ -1293,7 +1293,7 @@ Automatic Minimum Size of Grid Items</h3>
 
 	For the purpose of calculating an intrinsic size of the box
 	(e.g. the box’s <a>min-content size</a>),
-	this value causes the box’s size in that axis to become indefinite
+	a <a>content-based minimum size</a> causes the box’s size in that axis to become indefinite
 	(even if e.g. its 'width' property specifies a <a>definite</a> size).
 	Note this means that percentages calculated against this size
 	will [=behave as auto=].