[css-align] Fix vague wording in baseline alignment for insufficiently-constrained cases.

Author: fantasai

css-align/Overview.bs

@@ -1416,7 +1416,7 @@ Aligning Boxes by Baseline</h3>
 	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
-	according to the rules of the <a>box alignment properties</a> in effect.
+	as specified by the relevant keyword (''baseline'' or ''last-baseline'').
 
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