[css-values-4] Rename <calc-constant> to <calc-keyword>, specify that the production can be expanded in some contexts.

Author: tabatkins

css-values-4/Overview.bs

@@ -4050,11 +4050,16 @@ Syntax</h3>
 	<dfn>&lt;calc-sum></dfn> = <<calc-product>> [ [ '+' | '-' ] <<calc-product>> ]*
 	<dfn>&lt;calc-product></dfn> = <<calc-value>> [ [ '*' | '/' ] <<calc-value>> ]*
 	<dfn>&lt;calc-value></dfn> = <<number>> | <<dimension>> | <<percentage>> |
-	               <<calc-constant>> | ( <<calc-sum>> )
-	<dfn>&lt;calc-constant></dfn> = e | pi | infinity | -infinity | NaN
+	               <<calc-keyword>> | ( <<calc-sum>> )
+	<dfn>&lt;calc-keyword></dfn> = e | pi | infinity | -infinity | NaN
 	<<rounding-strategy>> = nearest | up | down | to-zero
 	</pre>
 
+	In some contexts, additional <<calc-keyword>> values
+	can be defined to be valid.
+	(For example, in [=relative color=] syntax,
+	appropriate channel keywords are allowed.)
+
 	In addition, [=whitespace=]
 	is required on both sides of the <css>+</css> and <css>-</css> operators.
 	(The <css>*</css> and <css>/</css> operators can be used without white space around them.)