Improve ngdoc for params and returns.

Split `accepts` arguments into `parameters` and `postData` for POST/PUT requests.

When `returns` is a single root argument, include its description in the `@return` comment.

Fix indentation where it was not consistent and/or caused different markdown output than intended.

These changes should improve the documentation of `User.login`, especially after strongloop/loopback#163.

/to @ritch please review.

The diffs of the code-gen template are rather difficult to read. Suggestions on how to make the template easier to read are very welcome.