[css-color-4] Clarify intended use of the parse a CSS <color> algorithm

Author: cdoublev

css-color-4/Overview.bs

@@ -1221,6 +1221,13 @@ Parsing a <<color>> Value</h3>
 	4. Return |used color|.
 </div>
 
+Note: This algorithm is not intented 
+to parse a CSS <<color>> value 
+specified in a CSS stylesheet
+or with a CSSOM interface,
+but in other places 
+like HTML attributes or Canvas interfaces.
+