usb: gadget: document a usb_ep_dequeue() requirement

Document the requirement that the request be dequeued and its
completion routine called before usb_ep_dequeue() returns. Also
fix some capitalization issues in the existing text.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>