Text docs - Update to improve readability for ellipsizeMode

Summary:
added context for easier understanding as 'ellipsizeMode' appears before 'numberOfLines' in https://facebook.github.io/react-native/docs/text.html

Explain the **motivation** for making this change. What existing problem does the pull request solve?

Upon first reading the Text docs, the ellipsizeMode function is confusing.  This update aims to remove the confusion.
Closes https://github.com/facebook/react-native/pull/11881

Differential Revision: D4533700

Pulled By: lacker

fbshipit-source-id: 948df16235a0ba02c14e630518bbd7261bc2b802

Libraries/Text/Text.js

@@ -98,6 +98,9 @@ const viewConfig = {
 const Text = React.createClass({
   propTypes: {
     /**
+     * When `numberOfLines` is set, this prop defines how text will be truncated.
+     * `numberOfLines` must be set in conjunction with this prop.
+     *
      * This can be one of the following values:
      *
      * - `head` - The line is displayed so that the end fits in the container and the missing text
@@ -110,8 +113,6 @@ const Text = React.createClass({
      *
      * The default is `tail`.
      *
-     * `numberOfLines` must be set in conjunction with this prop.
-     *
      * > `clip` is working only for iOS
      */
     ellipsizeMode: PropTypes.oneOf(['head', 'middle', 'tail', 'clip']),