[css-fonts-4] Add some more explanation to font-palette:light|dark

Author: litherum

css-fonts-4/Overview.bs

@@ -6221,6 +6221,9 @@ Animation type: discrete
 
 </dl>
 
+Note: The names 'light' and 'dark' describe the <i>background</i> the color palette is usable on,
+      not the colors in the color palette themselves.
+
 <div class="example">When dark mode is in effect, use a dark mode palette
 	<pre>@media (prefers-color-scheme: dark) {
 		.banner {font-palette: dark;