[css-grid-1] Add an explanation to the Changes addition from the previous commit.

Author: tabatkins

css-grid-1/Overview.bs

@@ -4920,6 +4920,15 @@ Minor Changes</h4>
 			See [[#serialize-template]].
 
 		<li id="change-2017-grid-baseline-search">
+			Properly specified that the "use baseline-aligned items if possible"
+			and "use first item that shows up, even if not in the first row"
+			conditions properly combine together,
+			so if there's nothing in the first row
+			but a baseline-aligned item in the second,
+			we'll use that
+			(rather than just taking the first item in that row).
+			Part of <a href="https://github.com/w3c/csswg-drafts/issues/3645">Issue 3645</a>.
+
 			<blockquote>
 				<ol>
 					<li>