Mention issue with AOA and multiple gamepads

Android does not support multiple HID gamepads properly over AOA.

PR #5270 <#5270>

doc/gamepad.md

@@ -50,6 +50,9 @@ It does not use the scrcpy server, and does not require `adb` (USB debugging).
 Therefore, it is possible to control the device (but not mirror) even with USB
 debugging disabled (see [OTG](otg.md)).
 
+Note: For some reason, in this mode, Android detects multiple physical gamepads
+as a single misbehaving one. Use UHID if you need multiple gamepads.
+
 Note: On Windows, it may only work in [OTG mode](otg.md), not while mirroring
 (it is not possible to open a USB device if it is already open by another
 process like the _adb daemon_).