[selectors4] Add :placeholder-shown per WG resolutions <http://lists.w3.org/Archives/Public/www-style/2013Feb/0393.html>

Author: fantasai

selectors/Overview.src.html

@@ -1949,6 +1949,15 @@ <h3 id="enableddisabled">
   <code>display</code> and <code>visibility</code> properties have no effect
   on the enabled/disabled state of an element.
 
+<h3 id="placeholder">
+The placeholder-shown pseudo-class <code>:placeholder-shown</code></h3>
+
+  <p>Input elements can sometimes show placeholder text
+  as a hint to the user on what to type in.
+  See, for example, the <code>placeholder</code> attribute in [[HTML5]].
+  The <dfn id="placeholder-shown-pseudo">:placeholder-shown</dfn> pseudo-class
+  matches an input element that is showing such placeholder text.
+
 <h3 id="checked">
 The selected-option pseudo-class <code>:checked</code></h3>