[PATCH] USB: kerneldoc for usbfs

So far as I know, "usbfs" was never documented ... so
here's a patch with some text I've had sitting around,
merging it into the other USB kerneldoc.

It should be accurate, down to the warnings about why
not to use several of the calls, though cross-review
with the code would be good too.  A notable omission
is any details on the async APIs.