[css-align-3] 'behaves as', because there are multiple stretch-like values. Also slightly rephrase for clarity. Fixes w3c#2986

Author: tabatkins

css-align-3/Overview.bs

@@ -1157,9 +1157,9 @@ Baseline Content-Alignment</h3>
 	it must also have a <dfn noexport>coordinated self-alignment preference</dfn>;
 	that is:
 
-	* If its <a>baseline alignment preference</a> is “first” (“last”),
-		the relevant computed <a>self-alignment property</a>
-		is either ''align-self/stretch'' or ''self-start'' (''self-end'').
+	* The relevant <a>self-alignment property</a>
+		behaves as either ''align-self/stretch'' or ''self-start'' (''self-end'')
+		given a <a>baseline alignment preference</a> of ”first” (“last”).
 		For this purpose,
 		the ''start'', ''end'', ''flex-start'', and ''flex-end'' values
 		are treated as either ''self-start'' or ''self-end'',