Maintain whitespace in propType documentation

Summary:
When looking at PropType information for certain props the formatting puts everything on one line which makes it quite hard to read. This PR ensures that the whitespace extracted from react-docgen is preserved.

For example the documentation for [Transforms](http://facebook.github.io/react-native/releases/0.34/docs/transforms.html#transform) goes from:

<img width="717" alt="screenshot 2016-10-01 17 32 23" src="https://cloud.githubusercontent.com/assets/691952/19015498/74aaa5c8-87fd-11e6-922d-3eef38cc6452.png">

to:

<img width="707" alt="screenshot 2016-10-01 17 32 30" src="https://cloud.githubusercontent.com/assets/691952/19015500/784521a4-87fd-11e6-9c23-717d1ee31921.png">

**Test plan:**

Check PropType documentation is being rendered correctly on other components.

**N.B.** more style tweaks could be made here to make things more readable (e.g. reducing line height, monospace font) but I'll refrain from making them here in case there are explicit designs for the docs.
Closes https://github.com/facebook/react-native/pull/10197

Differential Revision: D3967759

Pulled By: hramos

fbshipit-source-id: bf3d3b2032dc6e9f5f1acf3907ecb7f57a2c57be

website/src/react-native/css/react-native.css

@@ -1262,6 +1262,7 @@ div[data-twttr-id] iframe {
 .propType {
   font-weight: normal;
   font-size: 15px;
+  white-space: pre;
 }
 
 .compactProps .propType {