docs: Document --url and --unix-socket on the rc page

This page started talking about what commands you can send, without
explaining how to actually send them.

Fixes rclone#8252.

docs/content/rc.md

@@ -180,7 +180,7 @@ User-specified template.
 Rclone itself implements the remote control protocol in its `rclone
 rc` command.
 
-You can use it like this
+You can use it like this:
 
 ```
 $ rclone rc rc/noop param1=one param2=two
@@ -190,8 +190,23 @@ $ rclone rc rc/noop param1=one param2=two
 }
 ```
 
-Run `rclone rc` on its own to see the help for the installed remote
-control commands.
+If the remote is running on a different URL than the default
+`http://localhost:5572/`, use the `--url` option to specify it:
+
+```
+$ rclone rc --url http://some.remote:1234/ rc/noop
+```
+
+Or, if the remote is listening on a Unix socket, use the `--unix-socket` option
+instead:
+
+```
+$ rclone rc --unix-socket /tmp/rclone.sock rc/noop
+```
+
+Run `rclone rc` on its own, without any commands, to see the help for the
+installed remote control commands. Note that this also needs to connect to the
+remote server.
 
 ## JSON input