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: NClemens Buchacher <drizzd@aon.at>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1260,9 +1260,10 @@ help.autocorrect::
 	This is the default.
 
 http.proxy::
-	Override the HTTP proxy, normally configured using the 'http_proxy'
-	environment variable (see linkgit:curl[1]).  This can be overridden
-	on a per-remote basis; see remote.<name>.proxy
+	Override the HTTP proxy, normally configured using the 'http_proxy',
+	'https_proxy', and 'all_proxy' environment variables (see
+	`curl(1)`).  This can be overridden on a per-remote basis; see
+	remote.<name>.proxy
 
 http.cookiefile::
 	File containing previously stored cookie lines which should be used