[css-text-4] Say that preserve-auto should follow platform conventions

Author: frivoal

css-text-4/Overview.bs

@@ -79,6 +79,10 @@ White Space Collapsing: the 'text-space-collapse' property</h3>
 			and if yes where and under what condition,
 			is UA defined.
 
+			Both with regards to collapsing
+			and to soft wrap opportunities,
+			the UA should follow platform conventions.
+
 		<dt><dfn>preserve-trim</dfn>
 		<dd>
 			This value preserves white space and <a>segment breaks</a> as for ''preserve''.