[css-grid] Fill in TODO for an example of multiple shims.

Author: tabatkins

css-grid-1/Overview.bs

@@ -3719,7 +3719,14 @@ Resolve Intrinsic Track Sizes</h3>
 			If an item uses multiple intrinsic size contributions,
 			it can have different shims for each one.
 
-			Issue: Add example of multiple contributions/shims.
+			<div class=example>
+				For example, when the <a>grid container</a> has an <a>indefinite</a> size,
+				it is first laid out under min/max-content constraints to find the size,
+				then laid out "for real" with that size
+				(which can affect things like percentage tracks).
+				The "shims" added for each phase are independent,
+				and only affect the layout during that phase.
+			</div>
 
 			Note: Note that both [[css-align-3#baseline-align-self|baseline self-aligned]]
 			and [[css-align-3#baseline-align-content|baseline content-aligned]] items