interface.md: expand on --interface use

usingcurl/connections/interface.md

@@ -15,3 +15,19 @@ would like to "bind" your local end of the communication to, with the
 
     curl --interface machine2 https://www.example.com/
 
+We discourage use of a hostname in there, because how that forces curl to do a
+name resolve to figure out an IP address. If you cannot specify an interface
+name, consider using a fixed IP address.
+
+In addition, you can ask to explicitly use and IP address or interface name to
+prevent curl from having to guess. Do this by prefix the string with `if!` for
+interface name or `host!` for an IP address. Like this:
+
+    curl --interface "host!192.168.0.2" https://www.example.com/
+
+    curl --interface "if!eth1" https://www.example.com/
+
+You can even specify a specific IP and on a specific interface by providing
+both like this:
+
+    curl --interface "if!eith1!192.168.0.2" https://www.example.com/