[css-fonts-4] Clarify description of font palettes

Based on feedback from an external contributor.

Author: litherum

css-fonts-4/Overview.bs

@@ -4401,7 +4401,7 @@ In these fonts, colors are referenced by index when describing the geometry of e
 These indices are resolved within a current active palette
 using a lookup table present inside the font.
 However, many fonts contain multiple palettes,
-each containing a set of complimentary colors
+each containing a set of complementary colors
 chosen by the font designer to provide pleasing visual results.
 
 <pre class="propdef">
@@ -4450,11 +4450,16 @@ Animatable: No (see below)
 
 	<dt><dfn type><<palette-identifier>></dfn>
 	<dd>
-		This value identifies an author-define palette to use.
+		This value identifies an CSS-defined palette to use.
 		Users can define a palette by using the ''@font-palette-values'' rule.
 		If no applicable ''@font-palette-values'' rule is present,
 		this value behaves as ''font-palette/normal''.
 
+        Note: Such a CSS-defined palette may
+        be a reference to an existing palette already present in the font,
+        be a modification of an existing palette already present in the font,
+        or be an entirely new palette.
+
 </dl>
 
 Issue: Add examples and pictures.