[css-fonts-4] Make 'swap' use the same "tiny block period" as others

w3c#1361

Author: Litherum

css-fonts-4/Overview.bs

@@ -2134,7 +2134,8 @@ for example, forcing all fonts to have a ''0s'' <a>block period</a>.
 
 	<dt><dfn>swap</dfn>
 	<dd>
-		Gives the font face a ''0s'' <a>block period</a>
+		Gives the font face an extremely small <a>block period</a>
+		(''100ms'' or less is recommended in most cases)
 		and an infinite <a>swap period</a>.
 
 		Note: In other words, the browser draws the text immediately with a fallback if the font face isn't loaded,