[css-color] Add forwardToSameClass() guidance to basic 'how to define a method' rules.

Author: tabatkins

css-color/Overview.bs

@@ -3325,9 +3325,18 @@ Adding New Color-Manipulating Methods</h4>
 
 		<li>
 			Define a forwarding method for all other color classes,
-			by calling <code>CSSColor.forward("myMethodName")</code>.
-			(This default implementation is not very efficient,
-				but it automatically works for all color classes,
+			by calling <code>CSSColor.forward("myMethodName")</code>
+			or <code>CSSColor.forwardToSameClass("myMethodName")</code>.
+			Use <code>forward()</code> if the method returns a non-color value,
+			such as a number,
+			or if it purposely returns a color value of a particular type,
+			like a conversion function.
+			Use <code>forwardToSameClass()</code> if the method returns a color,
+			and the precise type of color returned doesn't matter,
+			as this will ensure it returns the same type of color as it was called on.
+
+			(These default implementations are not very efficient,
+				but they automatically work for all color classes,
 				including ones you define
 				or that a UA adds in the future.)
 

css-color/Overview.html

@@ -3382,11 +3382,20 @@ <h4 class="heading settled heading" data-level=15.5.1 id=api-adding-methods><spa
 
 		<li>
 			Define a forwarding method for all other color classes,
-			by calling <code>CSSColor.forward("myMethodName")</code>.
-			(This default implementation is not very efficient,
-				but it automatically works for all color classes,
+			by calling <code>CSSColor.forward("myMethodName")</code>
+			or <code>CSSColor.forwardToSameClass("myMethodName")</code>.
+			Use <code>forward()</code> if the method returns a non-color value,
+			such as a number,
+			or if it purposely returns a color value of a particular type,
+			like a conversion function.
+			Use <code>forwardToSameClass()</code> if the method returns a color,
+			and the precise type of color returned doesn’t matter,
+			as this will ensure it returns the same type of color as it was called on.
+
+<p>(These default implementations are not very efficient,
+				but they automatically work for all color classes,
 				including ones you define
-				or that a UA adds in the future.)
+				or that a UA adds in the future.)</p>
 
 		<li>
 			Optionally, define a more efficient version of the method