usb: Restore the reference to ch9.h

Keep the textual reference to ch9.h as it was prior to commit
caa93d9b ("usb: Fix up movement of USB core kerneldoc location").

As linux/usb/ch9.h does not contain comments anymore, explain
that drivers/usb/common/common.c includes such header and provides
declarations of a few utilities routines for manipulating the data types
from ch9.h. Also mention that drivers/usb/common/debug.c contains
some functions for creating debug output.

Fixes: caa93d9b ("usb: Fix up movement of USB core kerneldoc location")
Reported-by: Alan Stern <stern@rowland.harvard.edu>
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20210425153253.2542816-1-festevam@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/usb/usb.rst

@@ -109,16 +109,19 @@ well as to make sure they aren't relying on some HCD-specific behavior.
 USB-Standard Types
 ==================
 
-In ``drivers/usb/common/common.c`` and ``drivers/usb/common/debug.c`` you
-will find the USB data types defined in chapter 9 of the USB specification.
-These data types are used throughout USB, and in APIs including this host
-side API, gadget APIs, usb character devices and debugfs interfaces.
+In ``include/uapi/linux/usb/ch9.h`` you will find the USB data types defined
+in chapter 9 of the USB specification. These data types are used throughout
+USB, and in APIs including this host side API, gadget APIs, usb character
+devices and debugfs interfaces. That file is itself included by
+``include/linux/usb/ch9.h``, which also contains declarations of a few
+utility routines for manipulating these data types; the implementations
+are in ``drivers/usb/common/common.c``.
 
 .. kernel-doc:: drivers/usb/common/common.c
    :export:
 
-.. kernel-doc:: drivers/usb/common/debug.c
-   :export:
+In addition, some functions useful for creating debugging output are
+defined in ``drivers/usb/common/debug.c``.
 
 Host-Side Data Types and Macros
 ===============================