Add documentation on .focus and .blur

Summary:
<!--
Thank you for sending the PR! We appreciate you spending the time to work on these changes.

Help us understand your motivation by explaining why you decided to make this change.

You can learn more about contributing to React Native here: http://facebook.github.io/react-native/docs/contributing.html

Happy contributing!

-->

(Write your motivation here.)

(Write your test plan here. If you changed any code, please provide us with clear instructions on how you verified your changes work. Bonus points for screenshots and videos!)
Closes #15515

Differential Revision: D5648285

Pulled By: hramos

fbshipit-source-id: fc1e50ffd18cc234771c5f40f92549e7e87cd28e

Libraries/Components/TextInput/TextInput.js

@@ -96,6 +96,9 @@ const DataDetectorTypes = [
  * AppRegistry.registerComponent('AwesomeProject', () => UselessTextInput);
  * ```
  *
+ * Two methods exposed via the native element are .focus() and .blur() that
+ * will focus or blur the TextInput programmatically.
+ *
  * Note that some props are only available with `multiline={true/false}`.
  * Additionally, border styles that apply to only one side of the element
  * (e.g., `borderBottomColor`, `borderLeftWidth`, etc.) will not be applied if