usb: gadget: add doc to struct usb_composite_dev

Added documentation to new struct members for WebUSB:
* bcd_webusb_version
* b_webusb_vendor_code
* landing_page
* use_webusb
to avoid warnings in the build of htmldocs

Fixes: 93c47394 ("usb: gadget: add WebUSB landing page support")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jó Ágila Bitsch <jgilab@gmail.com>
Link: https://lore.kernel.org/r/Y95MRZZz3yC5lETB@jo-einhundertSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/usb/composite.h

@@ -432,6 +432,10 @@ static inline struct usb_composite_driver *to_cdriver(
  * @qw_sign: qwSignature part of the OS string
  * @b_vendor_code: bMS_VendorCode part of the OS string
  * @use_os_string: false by default, interested gadgets set it
+ * @bcd_webusb_version: 0x0100 by default, WebUSB specification version
+ * @b_webusb_vendor_code: 0x0 by default, vendor code for WebUSB
+ * @landing_page: empty by default, landing page to announce in WebUSB
+ * @use_webusb:: false by default, interested gadgets set it
  * @os_desc_config: the configuration to be used with OS descriptors
  * @setup_pending: true when setup request is queued but not completed
  * @os_desc_pending: true when os_desc request is queued but not completed