[css-align] Adjust wording defining first/last baseline set. Fixes w3c#1408.

Author: fantasai

css-align/Overview.bs

@@ -10,6 +10,7 @@ ED: https://drafts.csswg.org/css-align/
 TR: https://www.w3.org/TR/css-align-3/
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
+Previous Version: https://www.w3.org/TR/2017/WD-css-align-3-20170515/
 Previous Version: https://www.w3.org/TR/2017/WD-css-align-3-20170407/
 Previous Version: https://www.w3.org/TR/2017/WD-css-align-3-20170215/
 Previous Version: https://www.w3.org/TR/2016/WD-css-align-3-20160614/
@@ -1593,11 +1594,12 @@ Baseline Alignment Details</h2>
 <h3 id='baseline-export'>
 Determining the Baselines of a Box</h2>
 
-	The <dfn export lt="first baseline set|first baselines">first baseline set</dfn>
-	(and <dfn export lt="last baseline set|last baselines">last baseline set</dfn>) of a box
-	for a given axis
-	is the <a>baseline set</a>
-	nominally associated with the first (last) line of text within the box.
+	Each box,
+	for a given axis,
+	has a <dfn export lt="first baseline set|first baselines">first baseline set</dfn>
+	(and <dfn export lt="last baseline set|last baselines">last baseline set</dfn>)
+	that nominally corresponds to the <a>baseline set</a> of
+	the first (last) line of text within the box.
 	The <dfn export>alignment baseline</dfn> is one of these,
 	usually the dominant baseline associated with the <a>shared alignment context</a>.
 	(See the 'dominant-baseline' and 'alignment-baseline' properties in [[CSS-INLINE-3]].)