Explain text-class-high-speed a bit

Author: mvirkkunen

Cargo.toml

@@ -16,7 +16,12 @@ rand = "0.6.1"
 [features]
 # Use a 256 byte buffer for control transfers instead of 128.
 control-buffer-256 = []
+
 # Use larger endpoint buffers for highspeed operation (default fullspeed)
+#
+# Note: usb-device doesn't truly support high speed enumeration yet, so setting this will make
+# TestClass only compliant with high speed mode. It may still manage to be enumerated as a full
+# speed device, but the descriptors will be invalid.
 test-class-high-speed = []
 
 [[test]]