[css-align] Associate baseline tables with baseline-sharing group rather than alignment context (which can have multiple groups). Fixes w3c#1452

Author: fantasai

css-align/Overview.bs

@@ -1663,13 +1663,13 @@ Aligning Boxes by Baseline</h3>
 	that all belong to a single <a>baseline-sharing group</a>,
 	the [=alignment subjects=] are baseline-aligned as follows:
 
-	First, generate the [=alignment context’s=] baseline table
-	from its <a>first available font</a>
+	First, generate the <a>baseline-sharing group</a>’s baseline table
+	from the <a>first available font</a> of the group’s [=alignment context=]
 	and overlay also the mirror of this baseline table by aligning their central baselines.
 	These are the baseline “grids” to which the [=alignment subjects=] will align.
 
 	Next, align each [=alignment subject=] by its specified <a>alignment baseline</a>
-	to the [=alignment context’s=] baseline table
+	to the group’s baseline table
 	or its mirror, whichever matches the [=alignment subject’s=] <a>line orientation</a>.
 	Unless otherwise specified (e.g. via the 'alignment-baseline' property),
 	the <a>alignment baseline</a> is the <a>dominant baseline</a> of the [=alignment context=].