[css-fonts-4] fix #1669 palettes in compound fonts

Author: svgeesus

css-fonts-4/Overview.bs

@@ -5592,7 +5592,20 @@ User-defined font color palettes: The ''@font-palette-values'' rule</h3>
     and do not apply to document language elements.
     When a given descriptor occurs multiple times in a given ''@font-palette-values'' rule,
     only the last descriptor declaration is used
-    and all prior declarations for that descriptor are ignored.
+	and all prior declarations for that descriptor are ignored.
+
+	Each of these palettes are applied
+	to each @font-face that share a family name.
+
+	Note: In the case where a family name is shared
+	between multiple physical fonts,
+	(such as when a compound font is created, using `unicode-range`),
+	and if those fonts have different palettes,
+	the result of specifying a partial palette
+	using ''@font-palette-values''
+	is unlikely to be what the author intended.
+	In this case,
+	it is better to supply a complete palette definition.
 
     A given set of ''@font-palette-values'' rules
     define which author-defined palettes are available for use