http.proxy: also mention https_proxy and all_proxy

The current wording of the http.proxy documentation suggests that http_proxy is somehow equivalent to http.proxy. However, while http.proxy (by the means of curl's CURLOPT_PROXY option) overrides the proxy for both HTTP and HTTPS protocols, the http_proxy environment variable is used only for HTTP. But since the docs mention only http_proxy, a user might expect it to apply to all HTTP-like protocols. Avoid any such misunderstanding by explicitly mentioning https_proxy and all_proxy as well. Also replace linkgit:curl[1] with a literal 'curl(1)', because the former gets translated to a dead link in the HTML pages. Signed-off-by: Clemens Buchacher <drizzd@aon.at> Signed-off-by: Junio C Hamano <gitster@pobox.com>