Remove custom Guzzle client docs from feature

The information seems too detailed to be shown in Features, so have it in
Configuration only instead.
i would have kept a `.. tip::` here saying that if you need some headers on every request you should do a custom guzzle and link (with a `:ref:`, not the `:doc:` link). people reading here will just see "ah i can send some header, so lets just hardcode (or inject) that header everywhere i use the proxy client or cache manager"