[css-color-4] Clarify sRGB and non-sRGB spaces

Author: svgeesus

css-color-4/Overview.bs

@@ -237,8 +237,8 @@ Resolving <<color>> Values</h3>
 
 	Issue(3845): When should channel-clamping occur?
 
-<h2 id='numeric-rgb'>
-RGB Colors</h2>
+<h2 id='numeric-srgb'>
+sRGB Colors</h2>
 
 	CSS colors in the sRGB color space are represented by a triplet of values--
 	red, green, and blue--
@@ -248,6 +248,9 @@ RGB Colors</h2>
 	but is also useful for specifying colors on other types of devices, like printers.
 	(See [[COLORIMETRY]].)
 
+	CSS also allows the use of non-sRGB colorspaces,
+	as described in [[#predefined]].
+
 	CSS provides several methods of directly specifying an sRGB color:
 	[=hex colors=],
 	''rgb()''/''rgba()'' notation,