Documentation Improvements: Variables/Helpers

## Check List

Please check followings before submitting a new feature request.

- [x] I have already read [Docs page](https://hexo.io/docs/)
- [x] I have already searched existing issues

## Feature Request

New to Hexo and finding some inadequacies in some of the documentation.

- Under [Helpers List Tags](https://hexo.io/docs/helpers#list-tags) (and other `list_` helpers) the "style" option doesn't offer what one can use if one doesn't want the default "list". What other options are there?
- Under [Variables](https://hexo.io/docs/variables) a few of the variables are just listed as type "???" or "array of ???", including
- The `page.next` and `page.prev` vars (which seem to return some kind of page/post object, but I'm not totally clear the limits of what properties it has.
- The `page.tags` and `page.categories` return some kind of object, but I can't find any documentation about what vars those objects might have.