[css-text-4] insert not replace, is the default for text-autospace

See #318 #4246

Author: frivoal

css-text-4/Overview.bs

@@ -8039,6 +8039,9 @@ Automatic Contextual Spacing: the 'text-autospace' property</h3>
 			The specified spacing is automatically inserted
 			if there are no space characters of any kind ([=Unicode general category=] <code>Z</code>) already there.
 
+			If neither ''text-autospace/insert'' nor ''text-autospace/replace'' are specified,
+			the behavior is the same as ''text-autospace/insert''.
+
 		<dt><dfn>replace</dfn>
 		<dd>
 			The specified spacing is automatically inserted