Mention exclusive_caps mode in v4l2 documentation

PR #4435 <https://github.com/Genymobile/scrcpy/pull/4435>

Signed-off-by: Romain Vimont <rom@rom1v.com>

doc/v4l2.md

@@ -21,6 +21,13 @@ This will create a new video device in `/dev/videoN`, where `N` is an integer
 (more [options](https://github.com/umlaeute/v4l2loopback#options) are available
 to create several devices or devices with specific IDs).
 
+If you encounter problems detecting your device with Chrome/WebRTC, you can try
+`exclusive_caps` mode:
+
+```
+sudo modprobe v4l2loopback exclusive_caps=1
+```
+
 To list the enabled devices:
 
 ```bash