HID: Document memory allocation properties of report_fixup()

HID: Document memory allocation properties of report_fixup()

The memory pointer returned by the report_fixup() hook does not get
freed by the caller.  Instead, report_fixup() must return (in return
value and *rsize) a memory buffer with at least the same lifetime as
the input buffer (defined by rdesc and original *rsize).

This is usually achieved using one of the following techniques:

* Returning a pointer and size to a sub-portion of the input buffer
* Returning a pointer to a static buffer
* Allocating a buffer with a devm_*() function,
  which will automatically get freed when the device is removed.

Signed-off-by: Günther Noack <gnoack@google.com>
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>