[css-align-3] First crack at fixing abspos center alignment, w3c#4659.

Author: tabatkins

css-align-3/Overview.bs

@@ -1726,9 +1726,15 @@ Effects on Sizing of Absolutely Positioned Boxes with Static-Position Insets</h3
 	wherever the 'direction' property of the <a>static-position containing block</a> is referenced,
 	instead reference the value of the 'align-self' or 'justify-self' property
 	(whichever is defined to apply to the relevant axis),
-	treating ''justify-content/normal'' as ''start''
+	treating ''justify-self/normal'' as ''start''
 	and any <a>distributed alignment</a> value as its <a>fallback alignment</a>.
 
+	In the case of ''justify-self/center'' alignment,
+	the [=available space=] for the box is
+	double the distance between the center of the [=static position rectangle=]
+	and the closest edge of the [=containing block=]
+	in the relevant axis.
+
 	<figure>
 		<img src="images/place-content-abspos.svg" width=480 height=330
 		     alt="Start alignment sizes into the space between