Link to httpClient documentation

So searching the repo we don't seem to link anything https://docs.guzzlephp.org/ should we update other references for consistency?

It is true that the link is not there but is it needed? Guzzle is rather easy to search for. And like #3 implies this would be setting an example that @see should be in the doc block for similar situations. That could become a burden. I think this is a won't fix.

We shouldn't have to search for Drupal documentation or figure out from a list of search results which one is the version to refer to.