Make eldoc-mode fully subsume haskell-doc-mode

In light of issues mulled over in #649 and #820 `haskell-doc-mode` should be removed, functionality should be put in `eldoc-mode`. `eldoc` actually works already with `haskell-mode`:

``` el
(set (make-local-variable 'eldoc-documentation-function)
'haskell-doc-current-info)
```
- [ ] find out if the above snippet is all that is needed for `eldoc-mode`
- [ ] implement what is missing
- [ ] write some coverage tests
- [ ] remove functionality that duplicates `eldoc-mode` from `haskell-doc.el`
- [ ] remove mentions of `haskell-doc-mode` from texi
- [ ] remove mentions of `haskell-doc-mode` from wiki
- [ ] remove mentions of `haskell-doc-mode` from all the random wikis and tutorials out there
- [ ] remove mentions of `haskell-doc-mode` from spacemacs (if needed)

Start with haskell-doc.el and the existing eldoc-documentation-function snippet, then inspect any current coverage tests. Determine what eldoc-mode still lacks for haskell-mode, cover it with tests, remove duplicated functionality, and update the texi, wiki, and other mentioned documentation references.

Written by the indexing model from the issue text.