Document that libusb_set_option may also return NOT_FOUND

If the platform is Windows but UsbDk is not available libusb_set_option
returns LIBUSB_ERROR_NOT_FOUND.[1] Adjust the documentation to reflect that.

[1] https://github.com/libusb/libusb/blob/26611eaa494ed9e077b5b0e1f999f5ae377de958/libusb/os/windows_common.c#L570-L578

Closes #735

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>

libusb/core.c

diff --git a/libusb/core.c b/libusb/core.c
index aadabd0..6aa1b0b 100644
--- a/libusb/core.c
+++ b/libusb/core.c
@@ -2189,6 +2189,7 @@
  * \returns LIBUSB_ERROR_INVALID_PARAM if the option or arguments are invalid
  * \returns LIBUSB_ERROR_NOT_SUPPORTED if the option is valid but not supported
  * on this platform
+ * \returns LIBUSB_ERROR_NOT_FOUND if LIBUSB_OPTION_USE_USBDK is valid on this platform but UsbDk is not available
  */
 int API_EXPORTED libusb_set_option(libusb_context *ctx,
 	enum libusb_option option, ...)

libusb/version_nano.h

diff --git a/libusb/version_nano.h b/libusb/version_nano.h
index cdfeb4b..631c36e 100644
--- a/libusb/version_nano.h
+++ b/libusb/version_nano.h
@@ -1 +1 @@
-#define LIBUSB_NANO 11517
+#define LIBUSB_NANO 11518