[css-color-hdr][editorial] clarification

Author: svgeesus

css-color-hdr-1/Overview.bs

@@ -535,6 +535,7 @@ Mixing Dynamic Range Limits: the ''dynamic-range-limit-mix()'' function {#dynami
 	</pre>
 
 	On a display with [=HDR headroom=] &lt;= 0, 
+	in other words, an SDR display,
 	this color will display as 
 
 	<pre class="lang-css">color(rec2100-linear 0.9 1.0 0.8)</pre>