[ot-docs] Radio URL instructions added to Native Install Guide for OTBR (#197)

User feedback requested this additional info.

Author: jrhodie

site/en/guides/border-router/build-native.md

@@ -65,6 +65,38 @@ Mar 01 05:46:26 raspberrypi otbr-agent[2997]: [INFO]-PLAT----: Host netif is dow
 
 > Note: If you receive an "InvalidArguments" error, make sure your RCP board is connected as `/dev/ttyACM0`. It happens that the device path may change to `/dev/ttyACM1` after flashing the RCP. Re-plugging the RCP should solve this.
 
+## Edit the Radio URL path
+
+The OTBR agent uses the `/etc/default/otbr-agent` configuration file for its
+startup parameters. This file contains the `OTBR_AGENT_OPTS` variable, which
+defines settings like the Radio URL. For example,
+`spinel+hdlc+uart:///dev/ttyACM0`.
+
+This configuration file is generated by CMake from the
+[`otbr-agent.default.in`](https://github.com/openthread/ot-br-posix/blob/74794140358dc91fc697361205ba3882f310e8aa/src/agent/otbr-agent.default.in)
+source file. It is then fed into the system service as the
+[service start parameters](https://github.com/openthread/ot-br-posix/blob/74794140358dc91fc697361205ba3882f310e8aa/src/agent/otbr-agent.service.in#L8-L9).
+
+To change the Radio URL path on your target system:
+
+1.  Open the OTBR agent configuration file in a text editor:
+    ```
+    $ sudoedit /etc/default/otbr-agent
+    ```
+
+1.  Locate the `OTBR_AGENT_OPTS` variable.
+
+1.  Locate the Radio URL portion of the string. For example, `spinel+hdlc+uart:///dev/ttyACM0`.
+
+1.  Change that string to your new path.
+
+1.  Save the file and exit the editor.
+
+1.  Restart the `otbr-agent` service to apply the changes:
+    ```
+    $ sudo systemctl restart otbr-agent
+    ```
+
 ## License
 
 Copyright (c) 2021, The OpenThread Authors.