docs: Suggest alternative command to get controller info

Signed-off-by: Alexander Tsoy <alexander@tsoy.me>

docs/BT_DONGLES.md

@@ -20,6 +20,7 @@ hci0:   Primary controller
         name jupiter
         short name
 ```
+If `btmgmt` command is not available, try `bluetoothctl mgmt.info` instead.
 
 
 ### Cambridge Silicon Radio