Fix showing documentation with company-doc-buffer.

Hi, this fixes two issues with `company-show-doc-buffer` (normally bound to `F1` when there are completions displayed). 

* It was still using the old fn for showing documentation (previously `cider-doc-buffer-for`, now `cider-doc-lookup`).
* `cider-doc-lookup` was always selecting the newly-opened `*cider-doc*` buffer, which was causing the buffer to be displayed twice (as `company-show-doc-buffer` also tries to do the same thing). I've added an option for this to `cider-doc-lookup` and updated its usage in `cider-doc`.