[css3-syntax] Add note about the undefinedness of at-rule values.

Author: tabatkins

css3-syntax/Overview.html

@@ -3009,6 +3009,10 @@ <h2 id=parsing><span class=secno>5. </span> Parsing</h2>
 
    <dd> An at-rule has a name, a prelude consisting of a list of component
     values, and an optional value consisting of an simple {} block.
+    <p class=note> This specification places no limits on what an at-rule's
+     value may contain. Individual at-rules must define whether they accept a
+     value, and if so, how to parse it (preferably using one of the parser
+     algorithms or entry points defined in this specification).
 
    <dt><dfn id=qualified-rule0>qualified rule</dfn>
 

css3-syntax/Overview.src.html

@@ -2404,6 +2404,13 @@ <h2>
 			a prelude consisting of a list of component values,
 			and an optional value consisting of an simple {} block.
 
+			<p class='note'>
+				This specification places no limits on what an at-rule's value may contain.
+				Individual at-rules must define whether they accept a value,
+				and if so,
+				how to parse it
+				(preferably using one of the parser algorithms or entry points defined in this specification).
+
 		<dt><dfn>qualified rule</dfn>
 		<dd>
 			A qualified rule has