[css-align-3] Link to CSSOM serialization rules. Fixes w3c#1415

Author: fantasai

css-align-3/Overview.bs

@@ -637,6 +637,7 @@ Baseline Alignment: the ''baseline'' keyword and ''first''/''last'' modifiers</h
 
     Note: Because they are equivalent, and ''baseline'' is shorter,
 	the CSSOM serializes ''first baseline'' as ''baseline''.
+	See [[cssom-1#serializing-css-values]].
 
 	Note: For the somewhat-related 'vertical-align' property,
 	due to inconsistent design decisions in CSS2.1,