[css-color-4] Differentiated between out of gamut but physically realizable colors, and imaginary colors.

Author: svgeesus

css-color-4/Overview.bs

@@ -5394,6 +5394,15 @@ Note: This section provides important context for the specific requirements desc
 	Gamut mapping is the process of finding an in-gamut color
 	with the least objectionable change in visual appearance.
 
+	Some out of gamut colors correspond to real-world colors
+	(they could be physically reproduced)
+	while others are imaginary colors
+	(they lie outside the spectral locus
+	and thus would need more than 100% of a single wavelength)
+	and could never be physically realized.
+	Those colors tend to come from calculations
+	such as "make this color 100x as saturated".
+
 <h4 id="GM-clip">
 Clipping</h4>