Update AsyncStorage doc

Summary:
Relates to #8203 for AsyncStorage API update.

- Added a small example to the intro section.
- Added jsdoc format tags to show up class description, parameter descriptions.
- Word-smithed many of the method descriptions.

I also made a bug fix to the autogen. It wasn't handling the scenario where a method may have no parameters.

**Test plan (required)**

Wrote a small sample app to test the snippet added to the intro section.

Ran website locally: http://localhost:8079/react-native/docs/asyncstorage.html

![api_asyncstorage](https://cloud.githubusercontent.com/assets/691109/16329457/84f9d69c-3997-11e6-9e68-3a475df90377.png)

Ran changed files through the linter.
Closes https://github.com/facebook/react-native/pull/8396

Differential Revision: D3481783

Pulled By: JoelMarcey

fbshipit-source-id: ebc4b9695482ada8a3455e621534d2a7fb11edf4

website/layout/AutodocsLayout.js

@@ -650,7 +650,7 @@ var Method = React.createClass({
           </span> || ''}
           {this.props.name}
           <span className="propType">
-            ({this.props.params
+            ({this.props.params && this.props.params.length && this.props.params
               .map((param) => {
                 var res = param.name;
                 res += param.optional ? '?' : '';