Reduce confusion in testing documentation. (#7818)

* Reduce confusion in testing documentation.

Just wanted to add some clarity to this page and link to the react tutorial on Jest's website to give people more information. Also changed enzyme's definition from library to utility to help reduce the confusion. Hope this makes sense to everybody :)

See https://twitter.com/damusnet/status/780752042675597312

cc @lelandrichardson

* Update 10.4-test-utils.md

docs/10.4-test-utils.md

@@ -6,7 +6,7 @@ prev: two-way-binding-helpers.html
 next: clone-with-props.html
 ---
 
-`ReactTestUtils` makes it easy to test React components in the testing framework of your choice (we use [Jest](https://facebook.github.io/jest/)).
+`ReactTestUtils` makes it easy to test React components in the testing framework of your choice. At Facebook we use [Jest](https://facebook.github.io/jest/) for painless JavaScript testing. Learn how to get started with Jest through the Jest website's [React Tutorial](http://facebook.github.io/jest/docs/tutorial-react.html#content).
 
 ```
 var ReactTestUtils = require('react-addons-test-utils');
@@ -14,7 +14,7 @@ var ReactTestUtils = require('react-addons-test-utils');
 
 > Note:
 > 
-> Airbnb has released a testing utility called Enzyme, which makes it easy to assert, manipulate, and traverse your React Components' output. If you're deciding on a unit testing library, it's worth checking out: [http://airbnb.io/enzyme/](http://airbnb.io/enzyme/)
+> Airbnb has released a testing utility called Enzyme, which makes it easy to assert, manipulate, and traverse your React Components' output. If you're deciding on a unit testing utility to use together with Jest, or any other test runner, it's worth checking out: [http://airbnb.io/enzyme/](http://airbnb.io/enzyme/)
 
 ### Simulate