[css-fonts-3] Mark that font-synthesis: small-caps should apply to all-small-caps too

Author: litherum

css-fonts-4/Overview.bs

@@ -468,7 +468,8 @@ at least as dark as its predecessor. Only values between 1 - 999 are valid, and
 	This property controls whether user agents are allowed to synthesize bold or oblique font
 	faces when a font family lacks bold or italic faces.  If 'weight' is not specified, user agents
 	must not synthesize bold faces. If 'style' is not specified, user agents must not synthesize
-	italic faces. If 'small-caps' is not specified, user agents must not synthesize small-caps faces.
+	italic faces. If 'small-caps' is not specified, user agents must not synthesize small-caps faces
+	nor all-small-caps faces.
 	A value of 'none' disallows all synthetic faces.
 
 	<div class="example">