Add doc for findTypeAt

This PR gives you the capability to return the doc of the expression type.

Here a sample with test-node.js file  : 

```javascript
/** base node package */
var http = require('http');
http
```
Tern request#type :

```json 
{"query":{"type":"type","file":"test-node.js","end":55,"docs":true,"urls":true,"types":true,"file":"test-node.js"}}
```

Tern response#type:

```json
{"type":"http","name":"http","exprName":"http","doc":"* base node package","origin":"node"}
```