usb: core: add urb->sgt parameter description

The parameter description of urb->sgt is lost, this will add it for
completeness.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20250711182803.1d548467@canb.auug.org.au/
Signed-off-by: Xu Yang <xu.yang_2@nxp.com>
Fixes: 488e6eaab8 ("usb: core: add dma-noncoherent buffer alloc and free API")
Link: https://lore.kernel.org/r/20250721104417.3442530-1-xu.yang_2@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/usb.h

@@ -1455,6 +1455,10 @@ typedef void (*usb_complete_t)(struct urb *);
  * @sg: scatter gather buffer list, the buffer size of each element in
  * 	the list (except the last) must be divisible by the endpoint's
  * 	max packet size if no_sg_constraint isn't set in 'struct usb_bus'
+ * @sgt: used to hold a scatter gather table returned by usb_alloc_noncoherent(),
+ *      which describes the allocated non-coherent and possibly non-contiguous
+ *      memory and is guaranteed to have 1 single DMA mapped segment. The
+ *      allocated memory needs to be freed by usb_free_noncoherent().
  * @num_mapped_sgs: (internal) number of mapped sg entries
  * @num_sgs: number of entries in the sg list
  * @transfer_buffer_length: How big is transfer_buffer.  The transfer may