[css-color-5]Clarify no RCS with legacy rgba() or hsla(), fix #7018

Author: svgeesus

css-color-5/Overview.bs

@@ -1085,6 +1085,30 @@ the allowed [=channel keywords=] are:
 	This takes the sRGB value of indianred (205 92 92) and replaces the red channel with 255 to give <span class="swatch" style="--color: rgb(255 92 92)"></span>  rgb(255 92 92).
 </div>
 
+Relative color syntax is only applicable to the non-legacy RGB syntactic forms.
+
+<div class="example invalid">
+	For example, this attempt to use the rgba
+	<a>legacy color syntax</a> would be incorrect
+
+	<pre>
+		rgba(from <span class="swatch" style="--color: darkblue">darkblue 16 32 b / 0.5</span> )
+	</pre>
+</div>
+
+<div class="example">
+	Instead, use
+
+	<pre>
+		rgb(from <span class="swatch" style="--color: darkblue">darkblue 16 32 b / 0.5</span> )
+	</pre>
+
+	This takes the sRGB value of darkblue (0 0 139) and replaces
+	the red, green and alpha channels to give
+	<span class="swatch" style="--color: rgb(16 32 139 / 0.5)"></span> rgb(16 32 139 / 0.5)
+
+</div>
+
 <h3 id="relative-HSL">Relative HSL colors</h3>
 
 The grammar of the ''hsl()'' function is extended as follows:
@@ -1117,6 +1141,8 @@ the allowed [=channel keywords=] are:
 	lightseagreen is hsl(177deg 70% 41%), so --complement is <span class="swatch" style="--color: hsl(357deg 70% 41%)"></span> hsl(357deg 70% 41%)
 </div>
 
+Relative color syntax is only applicable to the non-legacy HSL syntactic forms.
+
 <h3 id="relative-HWB">Relative HWB colors</h3>
 
 The grammar of the ''hwb()'' function is extended as follows: