user docs: Python documentation contains strange YXXY strings

[g:python_host_prog](http://neovim.io/doc/user/nvim_python.html#g:python_host_prog) and next variable documentation currently looks like

> ```
> To point Nvim to a specific Python 2 interpreter, set YXXYg:python_host_prog|:
> ```

: note YXXY. Link is absent.
