[css3-page] Clarify that f in specificity can only be 0 or 1

Author: SimonSapin

css3-page/Overview.html

@@ -1135,7 +1135,8 @@ <h3 id=cascading-and-page-context><span class=secno>5.4. </span>Cascading
 
    <ul>
     <li>Count the number of page type names (= <var>f</var>)
-     <p class=note>This is 0 or 1.
+     <p class=note> Given the syntax of page seletors, <var>f</var> can only
+      ever be 0 or 1.
 
     <li>Count the number of ‘<code class=css>:first</code>’ or ‘<code
      class=css>:blank</code>’ pseudo-classes (= <var>g</var>)

css3-page/Overview.src.html

@@ -819,7 +819,8 @@ <h3 id="cascading-and-page-context">Cascading in the page context</h3>
 
 <ul>
   <li>Count the number of page type names (= <var>f</var>)
-    <p class="note">This is 0 or 1.
+    <p class="note">
+      Given the syntax of page seletors, <var>f</var> can only ever be 0 or 1.
   <li>Count the number of ':first' or ':blank' pseudo-classes (= <var>g</var>)
   <li>Count the number of ':left' or ':right' pseudo-classes (= <var>h</var>)
 </ul>