Fixed ac-cider-popup-doc and modified readme.

The function `nrepl-send-sync-request` is renamed into `nrepl-send-request-sync` in cider 0.7.0

https://github.com/clojure-emacs/cider/blob/v0.7.0/nrepl-client.el#L723-744

And maybe some one want to use `ac-cider-popup-doc` to popup doc instead of `cider-doc`, so i modified the readme.

I agree that it became unnecessary to replace CIDER's own doc with Compliment's one. The former now is better as standalone documentation. I already removed `ac-cider-popup-doc` once, but brought it back because of the request, as backward compatibility for those who already use it like that. I don't think we need to encourage this approach in future.