[cssom] Advisements are normative. Tweak wording to match CSSWG discussion and put it where it'll be noticed. w3c#3814

Author: fantasai

cssom-1/Overview.bs

@@ -949,13 +949,9 @@ The <dfn method for=CSSStyleSheet>deleteRule(<var>index</var>)</dfn> method must
  <li><a>Remove a CSS rule</a> in the <a for=CSSStyleSheet>CSS rules</a> at <var>index</var>.
 </ol>
 
-#### Legacy CSSStyleSheet members #### {#legacy-css-style-sheet-members}
+#### Deprecated CSSStyleSheet members #### {#legacy-css-style-sheet-members}
 
-<p class=advisement>
   Note: These members are required for compatibility with existing sites.
-  Authors are encouraged to use the standard members defined above instead,
-  which are consistent with {{CSSGroupingRule}}.
-</p>
 
 <pre class=idl>
 partial interface CSSStyleSheet {
@@ -982,6 +978,12 @@ The <dfn method for=CSSStyleSheet>addRule(<var>selector</var>, <var>block</var>,
  <li>Return <code>-1</code>.
 </ol>
 
+<p class=advisement>
+  Authors should not use these members
+  and instead use and teach the standard {{CSSStyleSheet}} interface defined earlier,
+  which is consistent with {{CSSGroupingRule}}.
+</p>
+
 CSS Style Sheet Collections {#css-style-sheet-collections}
 ----------------------------------------------------------