[css-values] Add issue noting that unit algebra will be added to this level, so we can go ahead and get a FPWD done without waiting for me to port the concepts from Typed OM.

Author: tabatkins

css-values-4/Overview.bs

@@ -1611,6 +1611,11 @@ Mathematical Expressions: ''calc()'', ''min()'', and ''max()''</h3>
 	or other expressions, such as ''attr()'',
 	that evaluate to a valid argument type (like <<length>>).
 
+	Issue: Per WG resolution, [=math functions=] will be extended to handle arbitrary unit algebra,
+	like ''calc(1px / 1em)'' resolving to a <<number>>.
+	This is already fully specified in [[css-typed-om]];
+	it just needs to be converted over to raw CSS values for this spec.
+
 	<div class="example">
 		<pre>
 		section {