[css-grid] Specify that multiple intrinsic size contributions can produce multiple shims, and add a reminder note about self/content alignment being in separate baseline-sharing groups.

Author: tabatkins

css-grid-1/Overview.bs

@@ -3706,6 +3706,17 @@ Resolve Intrinsic Track Sizes</h3>
 
 			Consider these “shims” as part of the items’ intrinsic size contribution
 			for the purpose of track sizing, below.
+			If an item uses multiple intrinsic size contributions,
+			it can have different shims for each one.
+
+			Issue: Add example of multiple contributions/shims.
+
+			Note: Note that both [[css-align-3#baseline-align-self|baseline self-aligned]]
+			and [[css-align-3#baseline-align-content|baseline content-aligned]] items
+			are considered in this step,
+			but they live in separate <a>baseline-sharing groups</a>.
+			[[CSS-ALIGN-3]]
+
 			</ins>
 
 		<li>