doc/user: document effect of http_proxy envvar on curl plugin

doc/user.xml

@@ -1814,6 +1814,13 @@ run</programlisting>
           database.
         </para>
 
+        <para>
+          Note that unless overridden by the below settings (e.g. by
+          setting them to a blank value), general curl configuration
+          from environment variables such as http_proxy or specified
+          in ~/.curlrc will be in effect.
+        </para>
+
         <informaltable>
           <tgroup cols="2">
             <thead>