[css-color-4] clarify opacity prperty vs. opacity as part of color, for overlapping text

Author: svgeesus

css-color-4/Overview.bs

@@ -4245,14 +4245,21 @@ Transparency: the 'opacity' property</h2>
 	are treated <em>as a whole</em>;
 	any overlapping portions do not increase the opacity.
 
-	<figure>
+	<figure id="arabic-opacity-rendering">
 		<img src="images/joining-and-transparency.svg"
 		alt="overlapping glphs rendered correctly, and incorrectly">
 		<figcaption>
-			Correct and incorrect rendering of text whose glyphs overlap.
+			Correct and incorrect rendering of text
+			with an 'opacity' value of less than one,
+			whose glyphs overlap.
 		</figcaption>
 	</figure>
 
+	If separate opacity for each glyph is desired,
+	it can be achieved by using a color value
+	which includes opacity,
+	rather than setting the 'opacity' property.
+
 	If a box has 'opacity' less than 1,
 	it forms a <a>stacking context</a> for its children.
 	(This prevents its contents from interleaving in the z-axis