http.proxy: also mention https_proxy and all_proxy

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>