USB: core: Update kerneldoc for usb_hcd_giveback_urb()

USB: core: Update kerneldoc for usb_hcd_giveback_urb()

The kerneldoc added for usb_hcd_giveback_urb() by commit 41631d3616c3
("usb: core: Replace in_interrupt() in comments") is unclear and
incorrect.  Update the text for greater clarity and to say that URBs
for a root hub will always use a BH context for their completion.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/41eaae05-116a-4568-940c-eeb94ab6baa0@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>