[css-values-4][editorial] Add a note about font-affecting to make it less COMEFROM.

Author: tabatkins

css-values-4/Overview.bs

@@ -2097,6 +2097,10 @@ Font-relative Lengths: the ''em'', ''rem'', ''ex'', ''rex'', ''cap'', ''rcap'',
 	(The other font-relative lengths continue to resolve against
 	the element’s own metrics when used in 'line-height'.)
 
+	Note: Most properties defined in [[css-fonts-4]] are [=font-affecting properties=],
+	as is 'math-scale'.
+	(This isn't necessarily an exhaustive list.)
+
 	When used outside the context of an element
 	(such as in <a>media queries</a>),
 	the [=font-relative lengths=] units refer to the metrics corresponding