Add instructions for launching a specific camera when multiple cameras are connected in FAQ

Author: ob-yalian

docs/en/source/8_FAQ/FAQ.md

@@ -58,4 +58,18 @@ find_package(OpenCV REQUIRED)
 ### Why Are There So Many Launch Files?
 
 - Different cameras have varying default resolutions and image formats.
-- To simplify usage, each camera has its own launch file.
+- To simplify usage, each camera has its own launch file.
+
+### How to Launch a Specific Camera When Multiple Cameras Are Connected
+
+While the launch file did not explicitly specify which device to use. In that case, the driver will connect to the default device.
+
+You can check the serial number of your device by running:
+```bash
+rosrun orbbec_camera list_devices_node
+```
+
+Then launch with the serial number explicitly set, for example:
+```bash
+roslaunch orbbec_camera femto_bolt.launch serial_number:=CL8H741005J
+```

docs/zh/source/8_FAQ/FAQ.md

@@ -68,4 +68,18 @@ find_package(OpenCV REQUIRED)
 ### 为什么有这么多启动文件？
 
 - 不同的相机具有不同的默认分辨率和图像格式。
-- 为了简化使用，每个相机都有自己的启动文件。
+- 为了简化使用，每个相机都有自己的启动文件。
+
+### 多相机连接时如何指定启动某一个相机
+
+如果启动文件未显式指定要使用的设备，在同时连接多台相机时，驱动会默认连接到其中一个（默认设备）。
+
+可以先通过以下命令查看设备序列号：
+```bash
+rosrun orbbec_camera list_devices_node
+```
+
+然后在启动时显式指定序列号，例如：
+```bash
+roslaunch orbbec_camera femto_bolt.launch serial_number:=CL8H741005J
+```