[css-grid] Rephrase to make it clearer what the boundaries of the definition are. Closes w3c#507.

Author: tabatkins

css-grid/Overview.bs

@@ -3865,9 +3865,10 @@ Resolve Intrinsic Track Sizes</h3>
 
 					Otherwise,
 					set its <a>base size</a>
-					to the maximum of its items’ <dfn noexport>min-size contributions</dfn>:
+					to the maximum of its items’ <a>min-size contributions</a>.
+					The <dfn noexport>min-size contribution</dfn> of an item is
 					the outer size that would result from assuming
-					an item’s 'min-width' or 'min-height' value
+					the item’s 'min-width' or 'min-height' value
 					(whichever matches the relevant axis)
 					as its specified size
 					if its specified size ('width' or 'height', whichever matches the relevant axis) is ''width/auto'',