[css-syntax] Specify that the informal syntax description is non-normative.

Author: tabatkins

css-syntax/Overview.html

@@ -3956,6 +3956,8 @@ <h2 id=anb><span class=secno>6. </span> The <a
   <h3 id=anb-syntax><span class=secno>6.1. </span> Informal Syntax
    Description</h3>
 
+  <p><em>This section is non-normative.</em>
+
   <p> When <a href="#a"><var>A</var></a> is 0, the <var>An</var> part may be
    omitted (unless the <a href="#b"><var>B</var></a> part is already
    omitted). When <var>An</var> is not included and <a

css-syntax/Overview.src.html

@@ -2919,6 +2919,8 @@ <h2 id="anb">
 <h3 id='anb-syntax'>
 Informal Syntax Description</h3>
 
+	<p><em>This section is non-normative.</em>
+
 	<p>
 		When <var>A</var> is 0, the <var>An</var> part may be omitted
 		(unless the <var>B</var> part is already omitted).