[css-grid] Move 'distribute extra space' algo into a subsection for clarity. No normative changes.

Author: tabatkins

css-grid-1/Overview.bs

@@ -3859,10 +3859,20 @@ Resolve Intrinsic Track Sizes</h3>
 			set its <a>growth limit</a> to its <a>base size</a>.
 	</ol>
 
-	<p id='extra-space'>
-		To <dfn noexport>distribute extra space</dfn>
-		by increasing the affected sizes of a set of tracks
-		as required by a set of intrinsic size contributions,
+	Note: There is no single way to satisfy intrinsic sizing constraints
+	when items span across multiple tracks.
+	This algorithm embodies a number of heuristics
+	which have been seen to deliver good results on real-world use-cases,
+	such as the “game”̣ examples earlier in this specification.
+	This algorithm may be updated in the future
+	to take into account more advanced heuristics as they are identified.
+
+<h4 id="extra-space">
+Distributing Extra Space Across Spanned Tracks</h4>
+
+	To <dfn noexport>distribute extra space</dfn>
+	by increasing the affected sizes of a set of tracks
+	as required by a set of intrinsic size contributions,
 
 	<ol>
 		<li>
@@ -3940,14 +3950,6 @@ Resolve Intrinsic Track Sizes</h3>
 	and distributing the shims’ space as if it were
 	part of the shimmed items’ original size contribution.</p></ins>
 
-	Note: There is no single way to satisfy intrinsic sizing constraints
-	when items span across multiple tracks.
-	This algorithm embodies a number of heuristics
-	which have been seen to deliver good results on real-world use-cases,
-	such as the "game" examples earlier in this specification.
-	This algorithm may be updated in the future
-	to take into account more advanced heuristics as they are identified.
-
 
 <h3 id="algo-grow-tracks">
 Maximize Tracks</h3>