[css-flexbox-1] Linkify some inner/outer box references.

Author: tabatkins

css-flexbox-1/Overview.bs

@@ -2603,15 +2603,16 @@ Main Size Determination</h3>
 					starting from the first uncollected item,
 					collect consecutive items one by one
 					until the first time that the <em>next</em> collected item
-					would not fit into the flex container's inner main size
+					would not fit into the flex container's [=inner size|inner=] main size
 					(or until a forced break is encountered,
 					see [[#pagination]]).
 					If the very first uncollected item wouldn't fit,
 					collect just it into the line.
 
 					<p>
 						For this step,
-						the size of a flex item is its outer <a>hypothetical main size</a>.
+						the size of a flex item is its [=outer size|outer=] <a>hypothetical main size</a>.
+						<span class=note>(Note: This can be negative.)</span>
 
 					<p>
 						Repeat until all flex items have been collected into flex lines.