[css-pseudo] Add text for the ::file-selector-button pseudo-element.

Closes #5049

css-pseudo-4/Overview.bs

@@ -891,6 +891,22 @@ Placeholder Input: the ''::placeholder'' pseudo-element</h3>
   It's not clear how this should work, but it may be worth doing.
   See <a href="https://github.com/w3c/csswg-drafts/issues/2517">Issue 2417</a>.
 
+<h3 id="file-selector-button-pseudo">
+File Selector Button: the ''::file-selector-button'' pseudo-element</h3>
+
+  The <dfn>::file-selector-button</dfn> pseudo-element targets the ''&lt;button>''
+  inside an ''&lt;input>'' element with <code>type=file</code>, if the UA
+  renders such a button.
+
+  There is no restriction on which properties apply to the
+  ''::file-selector-button'' pseudo-element.
+
+  <div class="example" id="file-selector-button-example">
+    For example, the following example should show a green border around the
+    file selector button:
+
+    <pre>::file-selector-button { border: 3px solid green }</pre>
+  </div>
 
 <h2 id="interactions">
 Overlapping Pseudo-element Interactions</h2>