[PATCH v4] docs/system/devices/canokey: Document limitations on usb-ehci

[MkfsSion]

Suggested-by: Hongren (Zenithal) Zheng <i@zenithal.me>
Signed-off-by: YuanYang Meng <mkfssion@mkfssion.com>
---
  v4:
    Adopt Zenithal's suggestion of repharsing the limitation 

 docs/system/devices/canokey.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/docs/system/devices/canokey.rst b/docs/system/devices/canokey.rst
index 169f99b8eb..db33b59e9a 100644
--- a/docs/system/devices/canokey.rst
+++ b/docs/system/devices/canokey.rst
@@ -156,6 +156,11 @@ to it, for example
 
    |qemu_system| -device piix3-usb-uhci,id=uhci -device canokey,bus=uhci.0
 
+The other limitation is that this device is not compatible with ``usb-ehci``
+since the device only provides the full-speed mode. However, when a
+full-speed device attach to a high-speed port, ``usb-ehci`` would complain
+about speed mismatch.
+
 References
 ==========
 
-- 
2.36.1