[css-color-5] Use term "modern color syntax" when defining RCS

Author: Chris Lilley

css-color-5/Overview.bs

@@ -788,7 +788,8 @@ the color functions could only specify colors in an absolute manner,
 by directly specifying all of the color channels.
 
 The new <dfn export>relative color</dfn> syntax
-allows existing colors to be modified
+extends [=modern color syntax=]
+to allow existing colors to be modified
 using the color functions:
 if an <dfn>origin color</dfn> is specified,
 then each color channel can <em>either</em> be directly specified,
@@ -806,6 +807,7 @@ The precise details of each function's changes to accomodate [=relative colors=]
 but they all follow a common structure:
 
 * An [=origin color=] can be specified with a ''from <<color>>'' value at the start of the function.
+* The origin color and the relative color need not use the same color function.
 * If an [=origin color=] is specified,
 	the remaining arguments can either be specified directly, as normal,
 	be specified as a <dfn>channel keyword</dfn> 
@@ -816,8 +818,7 @@ but they all follow a common structure:
 	If the alpha value is omitted, however,
 	it defaults to taking from the [=origin color=]
 	(rather than defaulting to ''100%'', as it does in the absolute syntax).
-* Relative color syntax only applies to the modern
-	(space separated components, / separator for alpha) syntax.
+* Relative color syntax only applies to the [=modern color syntax=].
 	It <em>cannot</em> be used with <a>legacy color syntax</a>
 	and attempting to do so is an error.