[css-align] Remove unnecessary/confusing 'aligned subtree' term. Fixes #1414.

Author: tabatkins

css-align/Overview.bs

@@ -423,7 +423,7 @@ Baseline Alignment: the ''baseline'' keyword and ''first''/''last'' modifiers</h
 	then the <a>fallback alignment</a> is used.
 	For example, ''align-content: baseline'' on a block box
 	falls back to ''align-content/start'' alignment.
-	The [=fallback alignment=] is also used to align the [=aligned subtree=]
+	The [=fallback alignment=] is also used to align the [=baseline-sharing group=]
 	within its [=alignment container=].
 
     Note: Because they are equivalent, and ''baseline'' is shorter,
@@ -1734,7 +1734,7 @@ Aligning Boxes by Baseline</h3>
 	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=].
 
-	Position this <dfn export>aligned subtree</dfn> within the [=alignment container=]
+	Position the aligned <a>baseline-sharing group</a> within the [=alignment container=]
 	according to the <a>fallback alignment</a> associated with the
 	specified <a>baseline alignment preference</a>.
 	For first (last) <a>baseline content-alignment</a>,
@@ -1743,7 +1743,7 @@ Aligning Boxes by Baseline</h3>
 	and the [=alignment subject’s=] edge
 	to align the start (end) margin edges of all the [=alignment containers=]
 	in the [=alignment context=]
-	while maintaining baseline alignment within the [=aligned subtree=].
+	while maintaining baseline alignment within the [=baseline-sharing group=].