url: --noproxy option overrides NO_PROXY environment variable

Under condition using http_proxy env var, noproxy list was the
combination of --noproxy option and NO_PROXY env var previously. Since
this commit, --noproxy option overrides NO_PROXY environment variable
even if use http_proxy env var.

Closes #1140

docs/cmdline-opts/noproxy.d

@@ -9,3 +9,7 @@ effectively disables the proxy. Each name in this list is matched as either
 a domain which contains the hostname, or the hostname itself. For example,
 local.com would match local.com, local.com:80, and www.local.com, but not
 www.notlocal.com.
+
+Since 7.52.2, This option overrides the environment variables that disable
+the proxy. If there's an environment variable disabling a proxy, you can set
+noproxy list to \&"" to override it.

docs/cmdline-opts/page-footer

@@ -23,6 +23,16 @@ Sets the proxy server to use if no protocol-specific proxy is set.
 .IP "NO_PROXY <comma-separated list of hosts>"
 list of host names that shouldn't go through any proxy. If set to a asterisk
 \&'*' only, it matches all hosts.
+
+Since 7.52.2, this environment variable disable the proxy even if specify
+--proxy option. That is
+.B NO_PROXY=direct.example.com curl -x http://proxy.example.com
+.B http://direct.example.com
+accesses the target URL directly, and
+.B NO_PROXY=direct.example.com curl -x http://proxy.example.com
+.B http://somewhere.example.com
+accesses the target URL through proxy.
+
 .SH "PROXY PROTOCOL PREFIXES"
 Since curl version 7.21.7, the proxy string may be specified with a
 protocol:// prefix to specify alternative proxy protocols.