improve TextInput docs

Summary:
Add `phone-pad` as one of cross-platform values to `keyboardType`.
Closes facebook#8555

Differential Revision: D3515002

Pulled By: JoelMarcey

fbshipit-source-id: 4f30dd72e8e30905a8a5fb90d3f361fb58eb825e

Author: ianlin

Libraries/Components/TextInput/TextInput.js

@@ -190,6 +190,7 @@ const TextInput = React.createClass({
      * - `default`
      * - `numeric`
      * - `email-address`
+     * - `phone-pad`
      */
     keyboardType: PropTypes.oneOf([
       // Cross-platform