curl_easy_setopt.3: remove what auth types that work for CURLOPT_PROXYAUTH

The list was out of date and the paragraph already refers to the
CURLOPT_HTTPAUTH explanation. All the auth bits are explained properly
there.

It also removes the ambiguity for what the "added" phrase refers to.

This change based on pull request #85 on github

URL: https://github.com/bagder/curl/pull/85
Reported-by: gnawhleinad