Improved documentation for <TextInput clearButtonMode=...>

Summary:
Documentation only. Actual behaviour was never changed.

Created from Diffusion's 'Open in Editor' feature.

Reviewed By: sahrens

Differential Revision: D6869466

fbshipit-source-id: 6e964433bb2b04b288736a3f01244285bc8c3fe8

Author: shergin

Diff for: Libraries/Components/TextInput/TextInput.js

@@ -499,6 +499,7 @@ const TextInput = createReactClass({
     defaultValue: PropTypes.string,
     /**
      * When the clear button should appear on the right side of the text view.
+     * This property is supported only for single-line TextInput component.
      * @platform ios
      */
     clearButtonMode: PropTypes.oneOf([