Proscribe duplicates via '||' value syntax operator (#315).

spec/ttml2.xml

@@ -1893,7 +1893,7 @@ which corresponds to production [3] <code>S</code> as defined by <bibref ref="xm
   <item><p>concatenated (juxtaposed) terms mean that all terms must appear in the stated order, e.g., <code>a b</code> means that the terms
   <code>a</code> and <code>b</code> are present and the former precedes the latter;</p></item>
   <item><p>a vertical bar (<code>|</code>) separates two or more alternatives, of which exactly one must appear;</p></item>
-  <item><p>a double vertical bar (<code>||</code>) separates two or more options, of which one or more must appear in any order,
+  <item><p>a double vertical bar (<code>||</code>) separates two or more options, of which one or more must appear in any order without duplication,
   separated by <loc href="#style-value-lwsp">&lt;lwsp&gt;</loc>;</p></item>
   <item><p>open and close parenthesis (<code>()</code>) are used for grouping and resolving operator precedence;</p></item>
   <item><p>open and close square brackets (<code>[]</code>) are used to express character classes;</p></item>