[css-align] fix wording for synthesized baselines to avoid an ambiguous parse. fixes w3c#134.

Author: tabatkins

css-align/Overview.bs

@@ -1358,8 +1358,8 @@ Determining the Baselines of a Box</h2>
 	and align that baseline set to the given single baseline.
 
 	To <dfn local-lt=synthesize|synthesized export>synthesize baselines</dfn> from a rectangle (or two parallel lines),
-	synthesize the alphabetic baseline from the lower line
-	and the central baseline by averaging the positions of the upper and lower lines.
+	synthesize the alphabetic baseline from the lower line,
+	and the central baseline by averaging the positions of the two edges or lines.
 
 	Note: The forthcoming Inline Layout Module will define synthesis rules for baselines other than alphabetic and central.