fullDocs option for doc_comment

See [discussion](https://github.com/marijnh/tern/issues/411) about this.

Updated demo with default value for fullDocs to make it easy for users
to understand. Added documentation to manual.txt (I don't know how to run the Makefile to regenerate the manual.html file - I'm using Windows).
This is an issue I've always had with tern but I thought it would be a difficult fix because I never looked at the code. Now that I looked at the code I realised how simple it is to fix this! 

Example:
```javascript
/*** section header ***/

/** test comments */
function test(){}
```

Why would it ever make sense to display the first comment when  there are more than one? I have a lot of code similar to above and I end up with `section header` as my documentation for function `test` inside of tern. It makes sense to display the last comment *(Unless there is a use case I'm not aware of)*.